I'm "stuck" at 1920x1200 for now, but I still get great performance gains with SLi and 3-way SLi. I wouldn't say SLi is a waste at 1080p. Things may run fine with one card, but they run even better with two and you can add much more eye-candy like the many different forms of anti-aliasing.
If the SLI profile for a game is good, SLI can help the minimum frame rate. Moving the minimum frame rate at 1080p from 40 to 60 fps is worth it. Maximum frame rates are more often than not an unhelpful gauge of performance.