It's a casual meetup and you don't really have to RSVP. You can just show up if you find it easier.
The Twitter RSVP is a lightweight operation that gives everybody a general sense for who is coming. You can see names, faces, bios and a way to get in touch with people easily. There is no need to create a networking site where people setup accounts and profiles. That would be over designed and engineered for what is needed to organize this event.
Hope this answers your question :)
On Tue, Jul 31, 2012 at 5:10 AM, Meetup <[address removed]>
Why do we have to RSVP via twitter ONLY? Am I the only one who finds this annoying?