I’d rather not be forced to use my Twitter login for WP authentication. It’s nice when it’s an option, but to force it might raise privacy concerns, among other things. I would prefer to have the option at registration: use a Twitter login for authentication and get my Twitter account synced automatically, or create a standalone WP login, which can later be linked to my Twitter account if I so wish. (This has the added bonus of allowing me to associate with a different Twitter account at a later date if I want to change.)
I recently set up a site for baby pictures, and managed to give users the choice between FB login and a dedicated WP login. Maybe it’d be helpful? http://teleogistic.net/2011/07/building-a-baby-photo-site-with-wordpress/