It really depends, don't you think? I agree with the general idea of your comment, but this card is supposed to perform on par with the GTX460 or so, which is 256 bit and not much higher in the stack we have the HD6850 which desperately needed a 256 bit interface too. The 192 bit 460 is not much slower than the 256 bit one, but it IS slower and if this card is supposed to perform in the neighborhood of these cards (that's their claim) it could definately use more bandwidth than what 128 bit interface will offer. They will probably use much higher clocked memory tho.
Now, I'm always of the kind that thinks that bandwidth is not as important and thus I think that 128 bit for this kind of card is a good compromise, but it's going to be a limiting factor anyways. It all comes down to the fact that we (enthusiasts) tend to see mid-range cards as "just good enough" cards, and as such "good enough" specs are due, especially bandwidth. People who usually buy mid-range think differently though: they want the best they can get and want to see as few corners cut as posible. Of course avoiding cutting corners is far more difficult on lower segments if you want to keep them cheap, but... it's human nature to want more!
So while I don't completely agree with their logic, I can't refute it either. That's the bottom line of this long (and probably worthless) post.