A flash is not a substitute for fast glass and fast glass is not a substitute for flash.
Fast glass can not magically provide fill in a backlit scenario. Fast glass can not create a rim light or side lighting, and is not the answer to shooting macro.
A flash will not help you when you want to shoot the Aurora Borealis. A flash will not provide a shallow DOF, or faster and more accurate AF performance, or a brighter viewfinder image. A flash may not even be allowed whilst shooting at a wedding, or indoor sports or at a stage performance.
You have to decide whether the focal length range is more useful to you than the faster aperture. Having a flash is nothing to do with it - well, very little at any rate.
p.s. I used to have a 17-85 and replaced it with the 17-55 a year later. For my needs/tastes I found f/2.8 to be far more beneficial, and the constant max aperture a great convenience when shooting with manual exposure, as I mostly do. Variable max aperture zoom lenses drive me round the bend. My 100-400 (the only variable max aperture zoom I now own) is almost never set to apertures faster than f/5.6 because I don't want my exposures screwed up every time I adjust the framing of the shot. Of course, your needs and preferences may be quite different.
