Are there any sites with runnable Servo binaries? Or is it seriously just the engine with absolutely zero browser chrome?
(1) Browser UX research isn't part of the research agenda for Servo at this time. (That isn't to say we aren't interested in browser UX research at Mozilla, just that that isn't being done under the Servo project.)
(2) It helps to ensure that we are embeddable. We want Servo to be an embeddable Web rendering engine for people to use in their own projects.