It makes sense for multi-node machines, much like we do some tasks mostly on CPUs and others on GPUs within a single node. A processor like this makes much more sense on desktops and general-purpose servers, as most of the time my Xeon cores are doing things an Atom would be perfectly capable of doing at a fraction of the power consumed. This translates into more heat and more cooling. If you consider a Xeon Phi uses 300 Watts for 256 threads, this translates roughly to 1.2 W per thread, which is well within what I would expect from a very puny Atom core. Being able to power down most of my computer while, say, I write this, would be a very nice feature.