I do see your point Dan, but "life" does happen and people will have to back out (even when they have RSVP'd as a yes). Even weddings you can account for a 20% rate of people not showing up after they've RSVP'd yes.
That being said, I definitely can see the frustration when things are planned and cost money/lose money. I would say that MS needs to come up with a "pay-ahead" plan, (refunds negotiable -- decided upon by the admins/moderators)... this way if funds are exchanged, I think the turnout rate would be more accurate.