Oh please, it's not as if Tent wasn't entirely germane to the topic of conversation. People plug their stuff all the time on HN, and as long as there's disclosure and it's relevant, I don't see the problem.
Also, you're missing the fact that users aren't the ones building services. A service that doesn't attract any services is useless to users. I'm not saying that Persona fits that description, but it's a fair point to bring up in terms of the spectrum between compromise and ideological pureness.
Its not, really. Its more the absence of an undesirable cost -- which is an advantage to end users, only if they get the benefit, which is access to services. But if the services don't adopt it because alternative log-in schemes that offer them more (both in terms of pre-existing user base and data from that user base) then what it really offers users is a "single sign-on" with nothing to sign on to.
> Persona " can't offer the same [as Facebook Connect] incentives (access to user data)"
That sounds like a feature. Gathering ever-creepier amounts of personal information to serve marginally-more-effective ads is a losing game.