Yes, but if a 6970 is going to cost you £150, and a 7950 will cost you £220, then very few people will bother going xfire, when they can sell their card for £100, buy a £220 new card, have spent LESS and have similar performance from a single card solution that uses half as much power.
While demand for those who want to xfire will be there, if the cards are too expensive, there won't be any point and people will just make the move on to the new gen cards anyway.
Now if a second hand 6970(or more likely 6950) is £100-120, and you already had one card, then its more of an option, though personally I'd still move on.
It only really made sense last gen for people to pick up 2nd hand 5850/5870's for xfire, because they got so cheap and the 6970 wasn't much faster at all, but again look at that pricing, EOL pricing on 5850's, £110-120, second hand prices, even less, and that was when the next gen wasn't 80% faster