That doesn't prevent spying when you are using your computer, which could be really embarrassing if you are for instance looking at porn. Also, sometimes you want to leave it one when you aren't using it, like when you're downloading something.
On the whole, I think this discussion focuses on the wrong 'problem'. If someone can gain access to your computer, taking control of the camera is just one of a whole host of nasty activities they could get up to - the real problem is securing against remote access in the first place.