It has to do with what your bottleneck really is- and frame buffer usually isn't the issue. (for sure at 2GB per card)
But, you're an AMD fan, so you're pointing out one of very few areas the 7970 could possibly have an advantage and trying to make it seem like as big of a deal as possible. I understand that and don't blame you, it's sort of like you're a guy whose favorite baseball player bats .158 and averages 3 errors a game, but happens to steal a base half the time he gets on base. You post the equivalent of "Joe Baseball may not have the highest batting average, but you've got to have a player who can steal a base once he gets on!".
I had two 3GB GTX580s on my 57X10 rig before I got the two 2GB 680s, know what difference I've noticed? That the 680s are faster at everything. All the reviews I've seen of surround show the 680s as either better or equivalent to the 3GB 7970s for surround as well.
There might come a day when 3GB will be necessary, but I'll have different cards then. I don't know of many people who buy $500 cards and try to keep them multiple generations- usually people who buy high end are enthusiasts who want to try the new cards, sell the old ones for half what they paid, and move on. Besides getting the new features and more performance, the economics make sense. One generation card, still has re-sale and buyers. Two gen, may as well use it to hold down papers.
You seem to be speaking to some imaginary market of guys who buy very expensive video cards and plan on keeping them the rest of their life.