Well a bashguard is designed to slide over objects. If you were to put teeth on them and then try to slide over something, it will grab and throw you off the bike. Also, when you hit a rock it will grind the teeth off or shear them off.
The ones with half-teeth that I was referring to are shaped so that they will slide forward, but grab and pull forward when rotating. Kinda like a ratchet gear, sorry I can’t describe it better and I’m too lazy to embed a pic lol.
A rubber bashgaurd would certainly have the potential to fetch up on obstacles though.