They seem to have exactly positioned themselves in a spot that will ensure that nobody ever uses them.

Cloud databases are what you reluctantly use when you realize that your thing is going to need to scale big. Database.com is prohibitively expensive for anything but tiny datasets and low traffic. Oops.

So while it would be great for running your in-house inventory application, there's no compelling reason to migrate off your existing (possibly free) relational database infrastructure.

And while it would be be great to be able to handle tons of requests to tons of data at the low latency they quote, you can do that on AWS for 1/1000 the cost.