I really can't think of a single use-case where the Steam Machine is the right choice actually, unless your one and only wish is to make a donation to Gabe.
You can have the same performances in the same form factor with the same OS (but more upgradability) for less, or you can improve on any of those points for the same price.
This also remains true for Apple! I don't care about performance. At all. I write text in a terminal! I only care about the hardware working as expected, out of the box, without having to tinker with bullshit. And Steam provides a platform that fulfills this, every time!