For personal preference I fall on the side of a completely open room. Of course you sometimes get people accidentally stumbling in who do not know anything about the Mudcat Cafe, a few have been a pain in the neck, and have been rapidly "bounced" by the admin. Others have been fascinated and enchanted by the diversity and quality of the singing and have felt privileged to have spent time with those of us who were there. There have been visitors, used to the Karaoke groups on paltalk, who arrived with pre-recorded backing tracks, but when it was explained that the mudcat room was about live song and music, they stayed with us and listened with good grace.
The only drawback of the open room is that it needs someone to open it as admin, and you can sit on your own for a long time in an open room if everyone else is still at the pub (or just arriving home for their tea / getting up from their bed on another continent).
All in all I believe the benefits of an open room for general purposes outweigh those of the alternative. If you are shy, you can always log onto paltalk using a variety of assumed names. I don't care what you call yourselves as long as the conversation and song are fun!
Quack!!!!!
GtD