1) The game takes off and the $200/mo is affordable 2) The game doesn't take off and the 5mil requests is adequate.
The lock-in concerns are valid, but right now their API isn't so mature that it would take eons to build out the pieces I am using in a week or two.
As for the concern with them going out of business, they've raised a good bit of funding, and offer data dumps so you can migrate elsewhere if need be.
For me, right now the ease of development outweighs the risk (we'll see in time though...)