As far as security goes, you’re half-correct. Users still (always) need to be vigilant about whose apps they install. But when applications themselves start offering services to other content, the security burden on app developers increases.

Clearly, it’s not impossible, but it’s something developers need to consider. Who is going to be able to call your “saveLocal” function? How do you verify the caller’s identity? The level of verification required changes, depending on the level of functionality being exposed. How can your exposed function be abused? Caller-provided paths or filenames are extremely dangerous, as they can be used to give an attacker local access. And on and on.

All these concerns can be addressed. But development is likely to be slow, because the price of getting it wrong is so high.