It keeps pretty much all of the casual spammers out. It keeps the quality of meetups high since the organisers have to put a little skin in the game - but no so much skin to prevent even small groups using the service.
I can remember when they didn't charge. The service was worse.
For those who've not read it already, take a look at this story of when meetup decided to start charging decided to start charging http://37signals.com/svn/posts/2751-scott-heiferman-looks-ba...
I would rather join a technical group on LinkedIn with events instead of using Meetup. The community is already there and almost all of us have LinkedIn accounts.
LinkedIn: please fix groups, please fix events.
EDIT: Forgot about Google Plus and events. Another potential alternative to Meetup.