I dropped my Sigma 85mm f/1.4 while in Geneva a month or so ago. I had a UV filter on the front. The filter broke and the mounting ring was slightly bent making it difficult to remove however the front element of the lens was fine. BUT... the AF motor was stuffed and there sounded like some internal damage so it was stuffed. Ended up buying another one in Berlin 2 days later because I love the lens so much.

So yep - it can certainly protect your front element (I use a filter on all of my lenses), as can a lens hood (I use it on all but the Sigma 85mm f/1.4 because I love the crazy flare you get without the hood and shooting into the sun - most people don't like flare, I reckon it's your friend). But a significant impact will do more damage than just the front element.