They're cheap enough that my first thought was to pick up a couple and figure out what I'd do with them later, but the disadvantage here is that they're older boxes, which means they're going to be energy hogs (for the performance point, at least). That's what makes the eco pitch interesting - they're reducing e-waste, which is good, but energy efficiency on newer boxes (especially if you just need minimal performanc…
I agree, this is a dreadful idea. The CPUs are going to hog a whole load of energy (as are the motherboards). The HDDs are going to be seriously used and probably prone to complete breakdown. An ARM SoC would do this same job much faster probably at much less cost, with FAR less energy consumption.
And apparently Intel has the same thing with it's atom chip: http://arstechnica.com/gadgets/2015/01/intels-compute-stick-...
With high capacity flash getting fast-enough for real use, I don't see much of a need for spinning platters anymore.
It's seems like a shame to give up on all that old hardware, but technology marches on I suppose.