I don't like hoods, but I keep them on for protection and flare issues, etc. I once shot without a hood because I saw that Spencer didn't use them in one of his behind the scenes shots. Unfortunately, I happened to get lots of flare that day, and not the good kind that everyone else seems to get...
I agree with the other hood users. First and foremost, it's free. Secondly, stray light from specific angles does affect your picture in many ways as mentioned by "flash" - it helps keep unwanted light out.
As far as protection goes, this conversation had taken place on the Canon board a few weeks back and one person stated a good point - a damaged front element requires around 150-300 USD to repair anyway - that's the price of a good filter these days. So, whether you crack the front element or crack a 200 dollar filter, at the end of the day, you're still out of the same amount of money. Professionals should have membership with Canon repair programs and should have insurance for these things.
Moreover, another point that was made was that regardless of how nice the filter is, it WILL affect your picture in some way. Now, whether you can see it or not is a different story. Light bends whenever it passes through anything - it bends when it enters the atmosphere, it bends when it goes through liquid, and it bends when it goes through glass and plastic. Putting an extra piece of glass or plastic in front of the lens is affecting the light that goes into the lens (some filters make this obvious, some are more subtle) - but there's no denyign facts of basic physics. Granted, some people might care and some might not.
I, personally, decided not to use filters after trying UV filters from Hoya a few times.
