Im currently traveling and am using a 10-22, 17-55 and 70-200 f4 IS with a flash.
The 17-55 is on my camera most of the time, I have noticed though, when i want it to be wider, I swap to the 10-22 and shoot it much wider. Even though i could just simply shoot at 15mm on the 10-22 I find myself stepping forward and shooting more in the 10-12 range to get a different type of perspective for the shot. I also seldom notice the 55-70mm gap in zoom range. The 17-55 is also good enough to shoot at 55 then crop to 'zoom' in without noticable loss in quality.
So looking towards the future, do you see yourself wanting an UWA?
I believe that if you buy a 15-85 and then find yourself wanting wider than 15mm and get a 10-22 or something similar, you will then will then wish you got the 17-55 for its better low light performance.
To me, the difference between swapping lenses because you want wider than 17mm and wanting wider than 15mm is not really a point in favor of the 15-85.