mostly just three reasons..... a) haven't really played a heavily graphics intensive game since Dark Messiah, b) makes my other card run warmer than i'd like, c) kinda wanna upgrade to an 8800GTX, and possibly keep one of them as a spare, at least for a little bit.... just in case.
they both work perfectly fine. haven't really tested them separately except for the one that i got a day or a few earlier than the other when i submitted my 7800GT for the Step-Up program toward one of the 7900GT's, and bought the other from Newegg. Don't see any problems when running SLI with them.
Might look into shipping costs. don't see it being anywhere near cheap though..... at least not as if sending it from here (PA), cross-country to California.... even with 2nd day.