I remember when I just started playing kingdoms, I didn't have any money to spend on gold so I bought whatever items I could.
To get the full gear quest I only needed a weapon. I was roman and none of the roman weapons were cheap enough. So I bought a paladin hammer for I believe 200 silver.
Another argument would be that the bonus applies not only to your own troops but to all defending troops, so if you know your teammates already have phalanx spears but no druid staves then you might just want to buy that one for when you need it in def.