I have seen RAR on Nexus, but I wouldn't say that it's common, at least for Bethesda's games, which is where I've seen it.
Things may have changed, but I recall that yenc (for ASCII encoding), RAR (for compression and segmenting) and PAR2 (for redundancy) were something of a standard for binary distribution on Usenet, and that's probably the main place I've seen RAR. I think that the main selling point there was that it was just a format that was widely-available that supported segmented files.