I agree, BitBucket's use of OpenID doesn't make sense. It's contrary to my experience with other sites which accept OpenID.
First, it's odd that BitBucket asks for a user ID after authenticating with an OpenID URI. The OpenID URI is supposed to serve the role of a user ID.
Second, while I understand BitBucket's procedures require some extra authentication for some operations, having OpenID users enter is unusual. Perhaps there is a way under the OpenID protocol to re-authenticate before doing such operations.
Asking for the combination of a user ID and a password, right after authenticating an OpenID identity, is what reasonably raises the reaction, "WTF?"
And come to think of it, why should entering a password be the way BitBucket confirms that a user really wants to perform some serious operations? Are you really worried that someone would break into an account without knowing a password, but then would be stopped by another demand for a password? Many other systems simply use a confirmation dialog ("Are you sure? This cannot be undone. [OK] [Cancel]") for such situations.
And if BitBucket asks for a password because it doesn't trust OpenID authentication, then you are IMHO better off not accepting OpenID authentication at all, rather than glueing a password on top of it.