I wear my plate armor mainly because it looks awesome, the only other armor I rly like is the skutatos, but then you get shot by every archer. I think I am around 70 body armor, a little bit more i guess, and I can say horse archers without bodkins do no damage at all to me, headshots deal like 5-10%. Normal archers without bodkins only do serious damage on me when they use a longbow, but horn & tatar bow are basicly ineffective aswell.
So to a certain degree plate armor is quite useful, especially when most of the HA's are too greedy to get bodkins, but against pierce damage armor is just useless, all it does is slowing you down which makes you an easy target.
Oh yeah, before i forget this, mendro can oneshot both me and my horse with a jarid for each of us...