I think it makes perfect sense, from a logical perspective, to use this naming scheme.
Think about it: Cayman is their top-of-the-line chip, for their second-digit identifier in the name they can choose anything from 1-9. It's top of the line chip so it gets 9. Barts is second best chip, it gets 8. And so forth down the line.
As such it was the previous generations that didn't make much sense. Although I concede that it does muddy the waters for the naming scheme of their dual-GPU cards.
Also, I find it unlikely that there are 5870 owners out there that would be so dense as to not read reviews before buying a 6870. Very unlikely. Those sort of people don't own high-end gear. And if they do, then they deserve to get burnt by their own stupidity.