I'm leery of building anything on top of someone else's API. I built on top of an API while also keeping in active contact with the company to let them know what I was doing, going as far as talking directly to their legal department, and they still shut me down when they couldn't handle the volume of usage. And this is in a situation where every marginal use of their API made them money. Collectively I was going to make them revenue of (projected admittedly) $1.2M. Didn't stop them from shutting me down.

Long story short: Building on someone else's API is a recipe for disaster. Are there times when you have to? Sure. But you need to know what you are getting into.