(I'm not sure why I got downvoted into oblivion. I stated my honest opinion--I haven't signed up for a lot of services where I can't use the username I want.)
I can't speak for anyone else, but it seems exceptionally ridiculous to me that you'd make a choice of which technology to use based on whether or not your username is available. I don't know if I'd downvote anyone for that, but it did inspire a laugh out loud moment.