It is useful, it helps fund the infrastructure, it helps fund hackathons, it helps hosting hackers during the events, it helps bringing developers who would not make it because they wouldn't afford the transportation, it helps getting hardware needed to write new drivers, etc, etc, etc...

As far as I'm concerned, a single week of hackathon achieves far more than months of individual work on the side, and the hackathons take place because the foundation can bring many developers together in the same place at the same time thanks to the donations it receives :-)