it seems, that you have never installed the Extension. Because then you would not have asked the questions.
I'm working on migrating OpenStack's MoinMoin wiki to MediaWiki. They use OpenID locked to launchpad's provider. I did the same with the OpenID MediaWiki extension there. I've also installed the extension in a number of other places. I'm quite familiar with it.
There's a pretty major difference between installing the extension on simple sites and Wikimedia. I explained all the usability issues that would stop us from using it.
Basically no one has solutions to some of the usability issues I've mentioned, which is why you'll notice very few large site have OpenID enabled as a consumer, and the ones that do don't prominently expose it.
- Ryan