Well, I can argue the other side and tell you about a specific time they were useful against me. Playing MA Ulm I start invading MA C'tis. My heavy infantry/knights run into a fairly homogeneous mix of bog beasts, Sirrrushes (with a decent bless) and undead backed by poison slingers. The undead kept the Sirrushes from being swarmed while the bog beasts were a little slower and ended up being a little back from the front lines spitting (fairly ineffective, but hard to get to to kill). The Sirrushes were strong enough to kill off anything short of a major assault, but the prospect of breaking through the front lines with a large force and overrunning the bog beasts sent me back to regroup until I could field decent poison immune troops. Who wants to engage in a fight where the best you can hope for is that you can kill most of the enemy before everybody dies?
That's a very specific example, but you can extend the logic - go ahead and run over me with those F/W vans...who's cost effective now?
