I shoot a lot of indoor activities often in a grade school gym. This includes 7-8th grade basketball, school musicals, and fund raising events that feature a live band. Low light performance is a must. My main body is a 60D, which is very similar to the 7D for low light performance.
For indoor events, the EF-S 17-55 2.8 is an excellent compliment to the EF 70-200 2.8L IS II. I have both and use both for all of the above scenarios. With low light, I want my lenses to deliver sharp, corner-to-corner images at wide open and both lenses deliver. The 70-200 is superior for sports, but both are used heavily for other events. The long lens lets me stay in the background and isolate the subject more with the tighter perspective. But, for closer quarters, the 17-55 can't be beat.
I considered the 24-70 2.8L, but ultimately chose the 17-55 for the IS and I'm glad I did. If you're paying for a sharp lens, don't dull the image with camera shake. With low light candids, shutter speeds often live between 1/60 and 1/30. This range is generally fast enough for subject movement, but can still be affected by hand held camera movement even from the steadiest shooter.
In addition, I have found numerous times when I needed the extra-wide range between 17-24. You can't always backup in a crowd.
There are two “quirks” with 17-55. Some complain about dust, presumably from being sucked in through the vents near the front element. I use a Hoya HD clear protector to avoid this – a tough filter that doesn't introduce additional glare.
The second quirk is that some copies have a “hitch” in the zoom ring between the 20-24mm range. You will feel more resistance when the zoom ring approaches this range. When you're playing with the lens and trying to determine whether it will bother you, the hitch feels huge. But, when I'm using the lens, I forget all about it. Maybe video shooters will notice it more, but I'm so focused (sorry) on getting the exposure right and on the subject that I've never noticed the zoom hitch in the field.
A nice-to-have for the crop-sensor body is the 35 2.0. Now that I have the 17-55, my 35 isn't used quite as often, but it's nice to have when I need a faster shutter speed in low light – or want less DOF. It's known to be soft in the corners on FF bodies, but the 60D crops this out. I find it to be quite sharp wide open. It's also nice when you want to lighten the load for grab shots.
From me, an ideal indoor event setup is a 60D with a 17-55 2.8 IS, and a 7D with 70-200 a 2.8L IS II. Don't have the 7D yet, but soon.