Whilst I agree there can be a lot of rubbish around; we spent a hell of a lot of time learning about semantics/logic etc when I studied philosophy at university. Whilst you could argue it's not philosophy in itself, you certainly need it to understand philosophy.
Take the "Do you believe in God/I am an agnostic" question. I've heard it argued that if by "I'm agnostic" you mean "There possibly is a god" causes logical hassles (using possible-worlds semantics here) as (assumption) God is beyond possible worlds - hence the statement implies he exists necessarily.
But you can counter that by saying "I'm agnostic" means "I don't know" (which it does, literally!)
So I think semantics is pretty important; at least I'd like to think that the 1/3 of my course spent studying it was worthwhile!
(ps - replies about agnosticism
here please
. I'm sure semantics discussions are allowed in this thread though)