When I first started playing tennis again several years ago, before I had a network of people to play with, I would look for hitting partners on craigslist. I knew roughly what my level was, based on playing a friend who had a USTA rating. So I would look for people advertizing themselves as at that level. Ended up being pretty much a waste of time - I ended up meeting up with 4-5 people IIRC, and all were much weaker than I had expected.
Ironically, they were probably all around the same level as each other, so presuming they were looking for partners only on craigslist, it would have worked out well for them since it appears that whether by luck or design they are all using the same scale. But whatever scale was being used is not equivalent to NTRP, so it's misleading to those used to NTRP.
Point is, it's useful to have a common language, and for lack of anything more universal, NTRP is it.