As a productive tool, I can't think of a use at all.

For learning distributed computing, I think has some advantages over just running all of the nodes on a single powerful computer. It means you can't hide from doing things scalably -- i.e. network costs between your processes are real; if your load isn't well-distributed across your nodes the OS scheduler can't save you; etc.

It's uhh.. cool. Haha. I can't think of a good reason either, but we're all hobbyists here, right? It looks like a fun weekend project.

There was a great article a while back about a guy running a warehouse-sized bitcoin mining operation, using RPi's to coordinate the many thousands of custom chips doing the mining. The Pi was the cheapest full linux a system available at the time, and performance wasn't really an issue.

Fun and learning. Is that so bad?

It only serves two fronts; education and development. Education-wise, you need something dirt-cheap to play with. For developer, you need some intermediate stage to deploy your work to test before going after a big cluster.