Never rely on anyones API, building something with API is mashup/side-project not a startup/company.
I think a better statement is "Never rely on anyone's API unless you have a contractual agreement and SLA in place, and a backup plan for if/when the API goes away; building something on an API otherwise is a mashup/side-project not a startup/company."
Doubtlessly. I hate being in the thrall of a company in such a way too. It's dangerous to be dependent upon someone else's their API. But is the chance of great success lower than for with other businesses? Probably better to start a Twitter client than a to-do list site or something.