Even for illustrative work, where it can give you a good base, it still sucks, because for actual painters this step (thumbnailing) is actually the quickest; most of the time-consuming painting process is 'finishing', or 'detailing' the rough.
However, where it's great is in giving the ability to inexperienced people to paint well. The hard part of the painting is getting the lightning, color scheme, perspective right, but the finishing process is quite mechanical. So it could ease the outsourcing of some art assets creation.
Of course the originial craft to producing high quality output is still needed - and just one image is not going to be sufficient anyway.
As you mentioned, I can also think of giving lesser experienced folks the ability to tinker with scene setup, dimensions, ratios etc and get faster 'final' results, although the 'old school' approach to getting those right before actually detailing something is quite important.
You can already do that in 3D if the overall concepts have been decided and some basic assets are there. Just switching the textures, lightning conditions, and some predefined building blocks do basically the same thing as this algorithm, except it's already part of the pipeline, and it gives you much more since it's 3D.
The other thing is that these algorithms look indecently good when you see a thumbnail, but very bad if you're looking at it too closely. These cool 'concept art' pieces that look good are 90% of what people see, but they do not represent 10% of the concept art work; most of it are boring details of joints, how the blade is strapped to the costume, how windows open, unsexy stuff as can be (that you can't do with such algorithms).
[1] CNNMRF, Neural Style plus Markov Random Fields: https://github.com/chuanli11/CNNMRF