Heh, well that is a very personal matter, and no one can really answer that but you. It only matters if YOU think it matters. If you keep your Cube hidden it probably isn't as big a deal, first of all. If it is in the open, you must ask yourself if you mind a jet black bottom on your Indigo GameCube. I honestly think black looks good with all GameCube colors except for possibly Spice, and NOA seems agree. I just wanted a matching system myself. I can ignore the black broadband adapter, but GBP stands out a bit more ;-)
Also, I could be wrong, but I don't think Nintendo has ever gone on the record stating that only black GBPs will be available in America, so it's possible we may still see other GBP colors later, if not at launch.