A 'maybe' makes it difficult for planning, especially when food or other things that cost money are involved. I always answer with a 'yes' or 'no' for this reason (as I've hosted lots of things over the years).

I respond with a "maybe" when I want to attend but don't know if it's possible yet. If "maybe" isn't an allowed response, I guess I'd have to answer "no" to be safe, even though I really do want to attend.

That seems suboptimal to me.