The problem is the API is huge, and the documentation isn't great either, so you're supposed to learn best practice from the apps in the OS source tree.
The function you are talking about only returns a single directory and my users want to use both if available.
It really isn't a big deal. Use reflection to find what works at runtime, and live with it. That's the price of the flexibility which makes the whole thing attractive in the first place.
The big problem is trying to cover all bases from the start: you won't. Attack it like an old school PC shareware dev, and respond to customer requests.
There are far larger problems with Android than this kind of thing. That things on external storage basically bypass the security model is a much bigger concern.