This may sound rather petty but I find it really annoying when working in manual that you have set your exposure correctly, working at 17mm at f/2.8 and at some point change the focal length and then have to change you exposure settings again cos it won't stay at f/2.8..... Call it lazy, call it what you want, it just bugs me cos I don't have to think about it when using my 70-200 until I want to change the way I am working. Does that make sense, lol
Plus if I did get as far as doing work in darkish places, like say churches, the extra stop may come in handy if say working at 50mm, I know its not much but its something.
Plus I've found the 17-70 to be soft at the edges when wide open and wondered if the constant f/2.8's dealt with it better.
Thanks for your reply though,