Since the plug lacks a FPU it would not be a great option.
Even more of an issue is that the ARM is a 32-bit CPU. If you're not doing FLOPS on a cluster, you're either doing distributed I/O (and the plug doesn't have a hard drive or much memory) or integer ops, and there you'd like a minimum of 64 bits to play with. If you had a version of the plug with a FP or a big integer adder+multiplier built in, then you might be in business.
Even then, if you were buying 1000 of them you might want to ask for a different form factor so that you could rack mount them with centralized power and cooling. Perhaps even colocate lots of CPUs on a single board. When you really get to large scale computing, what costs you is not computation (that's picojoules) it's moving bits around on-chip (10s to 100s of picojoules) or off-chip (easily nanojoules).
