Eh, the user still has to go check their email to find whatever password has been assigned. That's an annoying step.

With the second example, I think it would be better to show both Facebook and Twitter logins. If the user authenticates with the wrong service, perhaps show a prompt encouraging them to authenticate with their other provider (Facebook or Twitter). Then you have both accounts authenticated and linked.

Still, in the grand scheme, all these signup and login schemes suck. Surely we can develop something better... ideas?