Praetorian "sucks" and inactives villages/players.

Praterion is the best defensive unit according to crop. The thing which accounts mostly is not how much a troop costs or how much defense it provides. the thing is how much troops will survive after defending. If you count that you will realize that units with high defense survive in large numbers while units with low defense even they are a lot can't survive as much as they should. Same sort of thing is applied in offense, If you notice EC's which are without any doubt the strongest attacking unit whats the purpose of placing them in game when 4 imperian provide more offense.

The thing which is most important is not just training of troops but their survival after battles as well. The units with high points in their respective fields survives.

The solution to the problem is indeed in the defense per crop as you will need about 5 Legionaries to give the same overall defense as 4 Praetorians. Initially you will indeed be better off by simply building Legionaries since you can buy 6 for the price of 4 but the issue is that these extra troops consume extra crop which indirectly will make Legionaries much more expensive than Praetorians for the same price.

To illustrate a large defense army of 6000 Legionaries (compared to the 4k Praetorians) will in a week eat enough crop to produce 988 more Praetorians, which is enough to bring the ratio to be beneficial to the Praetorian (6k vs 5k when the break even ratio is 5:4). If you waited 2 weeks more you would now have same numbers of both units for the same price.
We can see from this that despite the Legionaries being much more efficient on the initial purchase, their lower defense per crop values will eventually allow Praetorian to pass them in efficiency, the longer the army stands and defends the more the scale turns towards the Praetorian.

You also have to take into account that Praetorian defense stats are actually much better than Legionaries as Infantry makes up most of army strength in real battles.
As for the other stats you have to realize that the Praetorian is very specialized unit. He is designed for one purpose only and should only be built for that specific purpose. Hes not supposed to be sent out to raid or attack any more than the Imperian is supposed to be left to defend your village.

The role of the Legionaries is to be a one size fits all solution that works decently in every situation but isn't optimal in any (well besides Roman cavalry defense, hes pretty good at that), if you need immediate defense hes your go to guy and he servers you especially well during the early game thanks to his dual functionality but eventually you will want to switch from him to the more cost effective Imperian and Praetorian

Actually, if you only take the attack/building time and build 24/7, the gaul army has the highest ratio of infantry compared to cavalry (Due to long training time of their cavalry), Teuton is about 50/50 and Romans have higher cavalry-attack.
And I have also seen late-game armies of 40k Imperians and 40k EC, which means the cavalry attack of the whole army was about 3 times higher, meaning Praetorians would be even weaker defending against that army than Legionnaries.
In the other hand, I have also seen Teuton armies with 150k Clubswingers and no Cavalry at all, so there the Praetorian would greatly excel obviously.

Roman And Gauls Hammers contain a large amount of cavalries while Teutons dont. The general thing is that most Teutons, nearly all are offense players and have very large amounts of clubs due to their cheap and quick training. While one the other hand Romans and Gauls are found to be defensive and offensive as well. So, the main defense which will be required is against infantry. For which Praterion is good. The thing I dislike about Prets is their speed. I place them as standing defense cause they never reach on time..

If Prets are given a bit extra attack and carrying capicity then Roman will be my first choice and prets will be my army..

@Txus

Indeed the Legionnaire isn't a bad unit and it's very versatile but that doesn't make Praetorian suck. Praetorian is just a very specialized defense unit, he's excellent at defense and if you need defense he's the one you go for.
You might think Praetorian is bad but that is because you are comparing him in wrong things. An established Roman player should absolutely create Praetorians if he wants defense units and even most offense Romans eventually want some defense to build into the Wonder or just to beef up their own territory and the Praetorian is the unit of choice for both.

Different units with different uses

@Philbaaa

Siege is also counted as infantry so that adds to each factions infantry power.

Some stats for anyone interested:
If you assume a ratio of 1:1 infantry:cavalry, Praetorians start being more cost effective than Phalanx after 710 hours or about 29.6 days. At this point the cost per defence-point is equal between the two unit types if you consider the consumed crop. Until then Praets cost more for the same defence-value. After that they are cheaper.
For Spears that point is reached after 1669 hours or about 69.2 days. For Legios it is after 427 hours or about 17.8 days.

If you assume a ratio of 3:2 infantry:cavalry it is 11.6, 13.4 and 7 days for Phalanx, Spears and Legios respectively.

If you send your troops to someone else and dont provide the crop yourself, this does not apply. In that case Phalanx and Spears are simply more cost effective than Praets.

But this completely ignores the buildtime. Since Phalanx and Spears can be produced significantly faster than Praets, you have to produce Praets in more villages to get the same def-point production. This means more resources need to be spent on buildings and upgrades.

With an infantry:cavalry ratio of 1:1 again, both Phalanx and Spears amount to about 1.5 times the def-point production of Praets. So in theory one would need 1.5 times the number of def-villages. I did not calculate the cost of this, though.

Which is best surely depends on circumstances, e.g. what are you constrained by, if anything, resources to build or crops to feed? What are you mostly defending against or attacking? With Romans you cannot forget the effect of the horse trough. Given that reduces the crop cost of EIs by one third, I think it's an attractive unit to build - and from what I've seen I'm not the only one to think that way.

They have the best total defence/ crop of any unit (100) however when you consider the spearmen with similar defence (95) but much faster build rate and reduced construction costs spearmen probably beat Praetorians.

The infantry defence is very useful as siege attacks are often more infantry biased or balanced. For pure cavalry attacks you can rely on the wall.

For semi casual player, how good would pure legionnaires be? The reduced defence would be offset by not needing an army for robbers. I usually need 1 sword for every 6 phalanx in defence.

This discussion is missing one important point

On paper, Praetorians have extremly strong defensive stats and as it has previously been stated, it's high deff/upkeep makes it probably the most cost effective defensive unit in the game. However, since they are slow they are not really that great in active defence situation but they are amazing for more permanent defence such as large treasuries or WW villages. Particularly as those tend to favor horse defence (phalanxes, spears etc etc) with most armies (gaul/teuton) tending to be more infantry heavy.

However, the problem is that there is usually not a whole lot of em.

The biggest downside of praetorians is by far their slow production speed. Compared to the other races the amount of defence you can produce per village as a roman is very low (easily 1/3 of a gaul with phalanx+druids/haeds. This makes them very difficult to play at a high level as you simply can't build enough of them. For smaller players this is less of a concern as they typically struggle with upkeep rather than production speed. But as a reference, check the table below:

Your table is missing Phalanx only and Spearmen only. But what I actually came for to write is that unlike an hammer, production speed isn't that relevant for def units. You can build multiple def villages to make up for the speed. The only moment a (good) roman defender is in a bad disadvantage compared to a gaul or teuton one, is when he produces more resources than all of his def villages can consume for a reasonable long period. It's not that bad how it looks like at the first shot, because a roman can also build longer in every village before he's out of resources

Roman is the only tribe you would single build as a def player though

so probably right not to include Phals or Spears as a solo build on an account, as they should always be dual built inf/cav (again for the same reason you mentioned "speed building")

Unless for kings and dukes, which have endless tributes, you won't be able to train in every village with barracks level 20 all the time, as soon as you gathered some troops. Solo Phalanx/Spearmen is better in terms of strength per crop in these cases.