"I mentioned this somewhere a couple of days ago and folks said it was generally not well received, and just a glorified karaoke machine." That may be because Roland advertising has suggested that karaoke is where it's at.
R-Mix has the potential to be fantastic for film sound where it is routine to wish you had a tool which can remove a small background noise from location dialog.
R-Mix has the potential to rescue a drum track with a squeaky pedal by removing the squeak.
There's all kinds of possibilities... not to mention the potential to dig in to sounds to grab samples that may be a basis for some further sound design.
There are uses such as these that Roland hasn't promoted with much vigor. It just emphasizes using it for karaoke backing tracks and so the result is what it is.
I've been looking forward to a R-Mix that will load 24bit files. The current free standing version only works on 16bit files.