I would only consider one, because I cheaped out on my last 24-70 and bought the f/4 ISCanonFanBoy said:IS would have been a nice feature to have at the time of purchase, but I'm not going to shell out the $$$$ for a new one with IS as long as I have this one. People with deeper pockets, people replacing an old or broken lens, or people just getting into this lens might. Not me.
I agree with you, though: for the majority of us non-pro's not earning big bucks on that FL, the current 24-70/2.8 is a spectacular lens that produces photos that always impress me. It's hard to imagine replacing it just to get IS.
On the other hand, there are lot of new 24-70's sold, and if I were making a decision between IS and no IS, assuming similar IQ, that would probably be an easy choice if the price and size weren't drastically more.