I would probably have added an "allow maybe" checkbox to the event creation dialog, but "maybe" doing the polarising thing is actually better.
In that regard, we've taken a page from 37signals' book "Getting Real":
"The best software has a vision. The best software takes sides. When someone uses software, they're not just looking for features, they're looking for an approach. They're looking for a vision. Decide what your vision is and run with it."
Hope that kind of explains where we're coming from and why we haven't given creator's the choice of response types.