Originally I voted no, because I honestly don't think we need one. But I suppose it could be handy for actually talking about the map while in the server, so that we don't need to rely on voice chat in game. Such as what happened today, when one certain person wouldn't shut up and for the first 3 maps the mappers couldn't get any useful feedback. I'd have it password protected though, maybe just post it in the VIP lounge so plenty of people have access to it, but it isn't publicly viewable. People would be free to distribute the password to anyone they want of course, within reason.
As far as just hanging out in it and chatting, I really don't think we should do that, because so many new mappers come to the actual steam chat room looking for help, it would be silly for everyone to migrate from there to the Vent server.