I think it depends on the game. Maybe if you're playing angry birds yes. If I make a quality game with a bit of depth etc, I imagine the audience would be different. I would in fact like the option to have it played on Win XP in an accelerated form when possible, and besides, my game is not playable without webGL anyway. If users care enough complain I would be more than happy to walk them through how to deal with the option.
I guess I don't see why having such an option available can hurt. And not having it means that essentially C2 no longer supports Win XP for my game. Not that any of this matters since I just do this as a hobby and no one who isn't me will play this game, but in theory