Because, the 660Ti is a 680 with disabled cores, TMUs, and ROPs. It performs on par because it happens that some games are good at scaling. That is not true for a lot of games. With crossfire and SLI, there is a higher chance of graphical corruption. Fbarnet is right in that aspect, but a 680 is a better option because it can play most games on highest settings @ 1080P. If you arent playing on >1080P, dont SLI.Well, that about settles it.
I'm getting two 660 TI's for SLI. As someone mentioned above it is surprising that these two cards perform almost as well as the $1,000 cards and cost several times less.
I will be gaming at 1080P, but you and the poster before raise some good points. I'll do some more research on both options because I'm not going to be getting a new GPU until sometime in late summer/early fall (hopefully before either Xbox 720 and PlayStation 4 launches).