PMB: the SP campaign is REALLY weak. The teammates you get to pick from are pathetic - start up a skirmish game instead, where you can define all of the bots attributes, and what weapons you allow, they play much better then. I've never played the whole SP game, it really made the bots look bad.
Play them in MP mode, on high skill, and they are quite impressive on many levels. Try it and then compare.
HOWEVER - I will totally agree their grenade AI is pretty weak, no matter what skill level. Not that I'd boast, but Fritz's grenade AI is MUCH more intelligent. I never understood why they did the whole "grenade the spawn" bit at all either. ???
