Yeah, the returns start diminishing after a point, but with an appropriate monitor I think there are still big differences at settings that single cards cannot handle smoothly. AA is nice but in my opinion it's no substitute for high resolutions for most games. 1600x1200 with 4xAA is generally slightly faster than "pure" 2048x1536 on anything but the 7800 cards, but I always choose the latter option whenever possible. (although either setting is only practical in FS2 open, Deus Ex and a few other old games for me)
Also, a good portion of the people for whom this sort of thing is targeted have that Dell 2405, which is a 1920x1200 (60hz) LCD, so they are out of luck with Crossfire too.
That being said, you're right that it is a bragging rights thing in some cases. I've seen quite a few people with overclocked 7800GTX SLI setups who post and argue on hardware forums pretty much all the time, so it makes you wonder if they ever actually play games with those cards.
