One fact missing from the above that might be fairly pertinent, is that Three Rings currently derives most of their income from in-game items bought with real cash.
For example, their Puzzle Pirates game used to be straight subscription only. Then they opened up new servers based on tradable micropayment tokens instead, and it took off like a rocket. Not just in number of players, but also in revenue - apparently without greatly increasing overheads.
That isn't true RMT, but fills the same demand. I think if game developers go the way of creating true RMT markets, they're abdicating any moral position they might have had on keeping real cash out of the game, but also missing out on any of the potential benefits of abandoning that position. So I think it's a pretty dumb idea.