It's not about Android permissions.
To access the microSd with write access, apps have to use completely new functions, not the old standard File based functions.
And the process of granting access to the microSd card is complicated to the user, I don't expect many will be able to.
In short, this is another one of Google's screw-ups and I really not too keen on cleaning up this mess for them.
Now in terms of "request permission on first use, not at install time" -- this is as I wrote above, completely unrelated.
This one is not a priority since the app will have to ask for them on first launch anyway (because they have to do with the app's core features, like being able to see sender images in message lists), not really different from the current "pre-6.0" permissions where you grant them at install time.
I mean this is very very different from "A flashlight app with Send SMS permission".