You make a good point about the moaners going home and letting the rest of the guests have a good time, but ultimately it is the host who makes the guest list and keeps the party going.   

You do make the mistake of assuming that the moaners are the one with the problem.   Often a host will let an obnoxious guest run herd over the festivities. The ones that you are calling "moaners" may have a legitimate gripe and would certainly do themselves a favor by finding another shindig. It would be no loss to them for leaving such an awful affair.

True, the host can't be blamed for having the party turning dismal, but they are the ones footing the bill as well as have the authority to make changes.