re: "consistent user experience"

That's the thing, they just need to make the UX so badass that users will want to use your products instead of a 3rd party. Problem solved. Instead, they are just being lazy and squashing anyone who tries to innovate on their platform. Considering every feature on Twitter.com started on 3rd party clients, they're just shooting themselves in the foot.

But then again, that's assuming you believe it is about "consistent user experience", which I think it likely 5% of the real answer.