Interesting discussion here, so lemme add my 2 cents:
Until recently ive been using (and still use it sometimes) a cheap Sigma 70-300 f4/5.6 lens, which is definately a slow lens - plus it is almost useless below f8, because you get really soft focus. Usually it wasn`t a problem to use f8, but should there be some overcast weather or if you were in a forrest, you`d be screwed - you`d have to go to ISO 800 or higher (which produces A LOT of noise).
3 months ago (just after 2008 Rally Finland, so all tele photos until 2008 Barum rally are taken with the sigma) I bought a prime Canon 200/2.8, which works much better in not so ideal conditions. Maybe you wouldnt use f2.8 on a circuit, but on a rally, where many stages are in the forrests, it definately has a use. I can give you a reallife example:
http://rallyphotos.cz/photos/barum2008/44.jpg
this photo was taken with the prime lens @ f2.8 and ISO 400 - resulting in an exposure time of 1/500, which should secure an relatively sharp pic. If i wanted to take that photo with my old sigma, it wouldnt be possible - first of all, i`d have to go to f8 (as i said, anything below that gives really soft focus) and then i`d have to use perhaps even ISO 1600 (useless).
But don`t get me wrong - the Sigma is still quite a good lens and I still use it quite often. The prime Canon just makes it a lot easier for me in bad conditions.
And also, i LOVE DOF, so 2.8 gives really nice effect
About noise reduction - i very rarely use it, and if i do, i use the noise reduction built-in photoshop, so nothing special there.