I mean assuming you can keep roughly 1,000 people subscribed at $5 a month, then each year a game would pay out $60,000 dollars. If you get a loyal fan following then this would be a great revenue source. Once people are emotionally tied into a game then it can often keep them in the game for many, many years.
If you could keep a lot of the heavy work on client-side (not unreasonable for modern computers if you're producing a non-CPU intensive game by nature) so that your bandwidth and server needs stay low, then once you break-even you'd hopefully be able to keep a large chunk of that $60k.
Correctly done, I imagine an MMO could quite possibly be the best way an "indie" developer could make big money without a publisher taking a huge cut. I mean sure if you can get a publisher to sell the game for $20 or so and let them take most of it to help grow your users, but keep the monthly fees in your pocket.