CLAIM: I cannot stress enough how valuable and extremely underrated the Legionnaire is, especially in the early game and for governors.
Some people may make the argument that it is the swordsman that helps 'balance out' the phalanx and together a sword/phalanx army combo is better than a purely legionnaire one.
Full Circle: Legionnaires are amazing and uber ultra mega super duper underrated they are IMO the best single troop in the game and are a governor's best friend and should most definitely not be overlooked.
Sorry for the long post and possible headache I just really like Legionnaires and wish they got the love they really deserve.
You won't see a single experienced/seasoned player building legionnaires for offense.
You got a very very basic and fundamental thing wrong.
There are OFF and DEFF units. You attack with offensive units and defend with defensive units.
That there are some units that kinda form a bridge between the two does not mean that it is good to build them. For defense build pretos, for offense Imperians. Nothing in between.
Why? Because there are specializations in this game. Once you play in a Kingdom you play either Off or Deff. Hybrid approaches don't work. And by hybrid I don't mean the off player that builds a small deff for emergencies or the Deff player with a little farm off.
You have to play a role in your kingdom. And that role is playing Off or Deff. For Off you should be experienced and using gold actively. It is very difficult to play Off successfully without any gold usage.
Coming back to your Legionaires: They have 85 defense value. By that they are one of the worst single crop defense units. Pretorians have 100/crop and are by that the best single crop defense unit. Followed by Spearfighters. Gauls have Phalanx which is still okay, but not good. Gauls strength is mounted defense and by that they fit whole different roles.
Also at the beginning you need to protect against Teuton bash raiders. Those that willingly kill some units to get the juicy ress. Legionnairs are more expensive and have less defense value vs Clubs attack value. Usually those players have far superior troop counts and can kill your legionaires with little losses.
The only use is at the very start for hitting robbers and if you have no deff partner to exchange units to get a little anti cav defense into your preto defense.