I don't remember which book it was (maybe "Inmates are running the asylum"), but there was an example of dancing bear. It is amusing to see that bear is able to dance at all, but that is by no means are good dance.
Also, how many users are constantly switching between different platforms? If all I see is the lousy app on my platform of choice I could not care less about its capability to run on others platforms.
A similar quote I read was "A horse that can count is a remarkable horse, not a remarkable mathematician".
You are right that users don't care about having one app running on multiple platforms. Instead, it is the software developers who would benefit by not having to write the same app three times.
You seem to be assuming that an Android app running on a non-Android platform would be 'lousy'. Is that because an app designed for a touch-based tablet would provide a bad user experience when run on a desktop computer?
Also, how many users are constantly switching between different platforms? If all I see is the lousy app on my platform of choice I could not care less about its capability to run on others platforms.