RasPi is ~ 175 MFLOPS per unit (CPU only, discounting the GPU). So, this cluster works out to 11 GFLOPS, with 16GB of RAM for > $2500 USD.
For comparison, you could buy a motherboard, 32 GB of RAM, and an Intel i5 processor for $500 that will do over 20 GFLOPS.
So, it doesn't really stack up well from a price/performance standpoint. The value of these systems is more teaching students how to work with parallel code.
Yes, from a performance standpoint there's no reason to go after this type of system. No one is arguing this is a good system for production work.
The application of this is a teaching model. It's a lot easier to demonstrate parallelism gains on this type of platform. Scaling beyond a single ARM core is going to give you immediate performance benefits. Scaling further out to the entire cluster will continue to show returns.
With a single desktop, once you go beyond ~4 cores the gains will drop off too quickly. You just won't be able to see gains out to 64 threads on a single CPU, where on this you should.
It also doesn't hurt to have a quirky architecture to get students excited by. And yes, you could also spend some time discussing the architectural trade-offs and why this is not a cost-effective system for production use.
Click the arrow that is to the left of the commenter's name. It seems you are one of today's lucky 10,000: http://xkcd.com/1053/
I made a bad assumption because your account is almost one year old. I'm sorry. Now I see that information is not given anywhere in this site. I think this vote method was taken from Reddit.
[0]: http://www.calvin.edu/~adams/research/microwulf/