In a perfect world there would be no need for RMT, but then again, in a perfect world we would not have time to play MMOs since we would all be too busy driving our Ferrari's and banging lingerie models.
But in our imperfect world I do think that the games I know, WoW and Eve seem to do it correctly enough. As long as it is tied to cosmetic items such as mounts, pets, skins and such. I'm also in favour of Plex and WoW tokens, since they help to enlarge the playerbase. Those who have plenty of time to play, but are short on money can keep subscribed while those who have plenty of cash but little time can buy things quicker.
But once WoW starts selling 100k Artifact power for $5 or a full set of top tier raid gear for $100 or whatever is where I would draw the line. That said, I have previously advocated for them selling more bank slots for cash. I'd fucking buy that.