Rechan's last post hits it on the head.
the only counter I have: Is it really true that Trippy McDisarm is ONLY good at trips and disarms?
A normal fighter is good in all sorts of fights, usually has ranged and melee weapon. Heck, I always pack a dagger, just in case of grappling, so I can get an extra stab in (since sword can't be used).
In the same vein, spending feats in 3e, to be the best at Tripping, and grappling, there's still plenty of room to carry a ranged and melee weapon for when trips don't work. And build on a fighter or monk, with high enough combat related attributes, he should be fine just with weapons.
Honestly, if I was playing Trippy McDisarm, I would not stand around with my head up my arse everytime we encountered a non-humanoid that wasn't using weapons.
Since I don't have 4e, I'm not talking about 4e's rules. Tripping is a valid strategy. It isn't the same attack every round. You trip with what your opponent gives you.
Lastly, one must consider the DM. a) ALL attacks should be describe with a reasonable amount of flavor. b) each encounter should be varied, using different skills and comat styles. If your environment is varied, you PCs will optimize to beat the common threat. If it's varied, they will optimize for a variety of problems.
It's that simple.