plenty of reasons to use 2.8, though I admittedly don't use it as much these days. Was really wishing for my 24-70 (which I left at home) the other day at an event. Yes, I used flash some, but would have enjoyed a bit more natural light and 2.8 would have allowed me to shoot with a faster shutter speed. I have shot primes (I started out with camera that was around before you were born and zooms were expensive and not good) and I love the added versatility of a zoom.
The real trade-off for me is the portability/weight vs. focal length. For me in most cases the 24-105 wins on a full frame, but I tend to shoot more close-ups and less very wide shots. Back to the original poster's question, I'm really not sure what I'd choose. I suggested my nephew get the 17-55 and he's been happy with it. I always stuck to EF mount lenses as I had a goal of moving to full frame--I had the luxury of not shooting tons of very wide shots.
You're right... Maybe I was just being a prick. I'm sorry. I shoot primes for my $$$ shots and I keep the 24-105 on my 50D all of the time for a walk around. I never take my 1ds3 out of the studio and it's never seen anything that wasn't a prime. For a cropper I seriously suggest the 24-105 paired with the 10-22. I understand that neither gets you 2.8 but UWA slower shutters dont realllllly matter unless your subject is moving. All in all I would say that either choice would be a good one (unless you get a bad copy of the 24-70) and keep in mind that alot of people think they have a bad copy (myself included) due to user error ie. non IS on the long end. Anyway, sorry to take over your post/question. Happy shooting.