It would be nice to have a "Stand By" list for classes, events, etc. that have limited seating. This would allow people interested in taking a class to be notified of last minute cancelations.
Talking to a couple of teachers, they account for a couple of people who don't RSVP, to sit in. But I can't allow people who don't RSVP because they haven't read and consented to rules of the course. For me it can be a safety issue.
From a personal stand point, there are classes I don’t sign up for in advance because of an unpredictable schedule. Knowing someone canceled last minute may allow me to attend. Also, its a little rude, even in an “unformal” setting, to interrupt an ongoing class to count heads for seating availability.