It's all about the person using the situation. SLI's not really something you just "plug in and load drivers and go play". That may work for.. 60% of games but if we know what we're doing and willing to manually tune the SLI profiles, we can actually get near 90% of PC games to run using SLI and scale well, even if the nvidia profiles don't exist or aren't optimized correctly.
Just takes a little time and figuring out. It's not hard to do.
But I think the majority of people either don't know how to tune (no offense intended at you directly, guskline.. I'm generalizing everyone here), or can't be ass'ed in to bothering with it, and so SLI gets a bad rep.
It's actually very good and can benefit a large list of games if tuned right.