"I've never been a fan of TD-only leagues" - agreed.
I've got Westbrook as my #2RB in a performance based league with points for receptions also, and he has done very well.
The reason he isn't getting the rushing TD's is quite simply...he doesn't have to. Everything McNabb throws up, is being caught, and he is using a bunch of different receivers to do it (including Westbrook). If the passing game is struggling (a big IF at this point) you'll see a lot more of Westbrook. As of right now, it looks like they are going to use him just enough to keep the opponents secondary honest. Of course, with his speed, he's always a threat to bust one open. It's really just a matter of time.