True, but the half that there think there's no difference are wrong.
With SLI it's true that the two cards run at the speed of the slowest, but that's not the case with crossfire. Because it uses different methods of sharing the load across the cards than SLI, it is possible to get some benefit from have a faster card.
In practice, it's a little more complicated. I remember a review where they tested a 5850 and 5870. In some games and settings settings, it was better than a crossfire 5850 setup, but obviously never as good as a crossfire 5870.
However, in some games at some settings, the 5870+5850 setup was worse than the crossfire 5850. The reviewers theorised there was some extra overhead caused by the cards running at different speeds.
In either case, it was way, way better than a single 5870. So if you had a 5870, and had the opportunity to add a 5850 for a decent price, or vice-versa, it was still very much worth doing.
With the improvements in the 6900 generations scaling, this quirk may no longer be true.
