I tend to feel that if an event tends to "sell out", people will sign up without really thinking through whether they really are free that night.
Having said that I don't really agree with suggestion that people should just turn up. I think we want to encourage people to sign up meaning to come and to cancel if they can't.
Not sure that setting RSVP limit to 2 days encourages people who find they can't come to sign off.If you know on the day that someone else can use your place then you are more likely (IMHO) to make the effort. A message a few days before asking that people check they are really coming, and to remove themselves from waitlist if they are no longer able to come.
(I have travelled home only to find when I got back that I had been accepted in the last couple of hours!)