Education.
A cluster of Pis is far cheaper than a cluster of just about anything else, and you don't need raw power just to teach people with.
A cluster of Pis is far cheaper than a cluster of just about anything else, and you don't need raw power just to teach people with.
Granted, that's no substitute if you need something with physical I/O ports, but if you want to learn how to set up a real compute cluster, EC2 is a more realistic environment.
Those two aspects combined more than make up for the really trivial cost Pi's may cost over micro EC2 instances unless you are really interested in pinching pennies above other considerations.
Model A should be out in the next few weeks, which will make that $25.