For a side-project, that is great. But if you are building a company where you have multiple people looking to you to keep supplying the paycheck that pays their mortgage, that just seems irresponsible.

Notwithstanding that, why would you want to build something that could have the rug pulled out from under it at any point? I build stuff because I enjoy building thing. If it wasn't working in the market and fails, it is my fault, and I can live with that. If it is successful in the market and fails through somebody else's actions (that I should have been cognizant of), that just sucks.

Because the alternative is to build CRUD app startups like Basecamp, with no dependencies on any API or 3rd party data. Useful, no doubt, but a saturated market.

Saying "CRUD application" is like saying "Java application". It's an implementation detail and not an actual feature.

Tumblr is a CRUD application, yet it shares almost nothing with Basecamp.

Focusing on the technology does not make great products.

FWIW, I think there are a lot of potentially interesting applications that could be built on Twitter, but they've proven themselves hostile to the idea. I don't even trust them as an OAuth provider anymore.