For what game?
You forget that their cards can do 8X SSAA, which when playing HalfLife2 at high res is probably playable on even the GF9600GT, so the extra memory might still come in handy, especially if enabling HDR.
With CUDA thrown into the mix as well, there alot more applications for memory than just as a buffer for graphics.
While it's not something that 99% of users would have a need for, it's also not something people are forced to buy, so whether they get a 512MB model or a 2GB model it doesn't really matter, but having that choice is better than not having the choice iif someone found a use for it. It not like they're forcing people to buy it at a higher price, they offer both higher and lower memory options. It's like an overclocked version of a card, might be the same underlying hardware, but you pay 30% more for a changed bios versus your own overclock?
Since they buy memory in bulk, sometimes with the way the devices are configured they can slap on that larger Mbit per chip memory on pretty cheaply as left over for another line that requires that high bit per pin count (like a 256MB+ 64bit solution etc), so offering it as and option takes little effort on their part.
BTW, the more appropriate question... why do you care if they offer 2GB?
Are you jealous and feel you missed out, or do you need low GDDR prices for a pet project?