I can speak on this topic from experience.
I *used* to use a 17-55/60D combo at certain social events. While I loved it in reception-style settings (where I often had an external flash like the 580EX II), I was often disappointed with the results when I used available light (ex. a well-lit room). They would often be noisy and lacking detail, even when I opened up all the way to 2.8. It was far better than the Tamron 17-50 I was using with it before, however.
Fast forward to this year, when I sold the 60D after acquiring a 5D3 late last year, and therefore had to give up the 17-55. I replaced it with - you guessed it - the 24-70 II. After using both combos in similar situations, here are my thoughts:
- My keeper rate was generally higher with the 17-55/60D, primarily due to the IS.
- However, (generally speaking) the quality of the 24-70II/5D3 is significantly better than what I was getting out of the 60D. This is even after accounting for the higher ISOs I often have to use to get shutter speeds fast enough to prevent blur in the absence of IS (which surprised me).
- I find that in general, I'm happier with what I'm getting out of the 24-70 II/5D3, even if my hit rate is somewhat lower (I take more shots to compensate). The pictures are not only sharper and cleaner, but the background blur when shooting at 2.8 is sometimes "Wow". Sometimes I miss the IS, but honestly, I don't think I would ever go back to using a crop camera when shooting these types of events. This is where FF cameras really shine, to me.
ETA: Another note...24mm on the 5D3 is noticeably wider in my experience than 17mm is on 60D/7D.