If they're going with the platform strategy, I'll offer an analogy: Windows has basic features built in like Notepad and Calculator, but that doesn't mean others can come along and build a much better implementation of these programs.
I fear they're running the risk of creating a platform that's too "bland".
Their strategy isn't really very open though. With heavy rate limiting and lack of a decent auth mechanism it's pretty sucky. I really don't like having to store a user's password.
Once twitter have OAuth and XMPP I'll think of them as open.
