Actually, we tried that, and I'm happy to re-introduce it if you like (for SE only). Your thinking seems to mirror Jeff's :) the problems we saw were:
- it didn't give people to see what was available
- it was inconsistent to jump straight in
- often it simply wasn't the most appropriate room
Currently, instead, some of your site's rooms get an artificial boost into the first row of the results when coming from an SE site. Is that enough? It seems to offer a reasonable compromise of site-specific and "what is there".
Idea (edit): maybe we should highlight all of the SE site's rooms when coming from a SE site?
Edit edit: there's also te issue of login; we can do a silent-ish login on the room index, but not really inside a room.