The U&I apps are very deep and well thought out. But they represent a way of working which many aren't used to. I think that composing entire works in NRT takes a bit more discipline than most can imagine. It's not the immediate gratification of running loops and tweaking VSTi knobs, for instance. Also, the programs are meant to be used together, you can pipe images from Artmatic into Metasynth, images from Metasynth into Xx sequences.
Of course, many people probably use it as a novelty to sonify some picture and never use it in depth.