[tmurray]

It depends entirely on where your bottlenecks are. If the bottleneck is entirely within your node, then this isn't going to be compelling. If you're doing something that's very light on the resources within your node (serving static content, etc) and your bottleneck is some other system somewhere else, then these sorts of machines could be compelling purely from a space/power POV.

[jsn]

If your nodes are not bound on some local resource, you can as well just run them in virtualization containers on Xeon. The setup will be even more flexible than with (less powerful) ARMs.

[ricardobeat]

But not nearly as space/power-efficient.