This seems like a bad assumption, and a worse one going forward.
And right now, I end up having to find the right device to run that app I want to use instead of using the device that I like with the app that I like...
No, I'm not. I said "platform". A single platform can encompass multiple devices.
An Android user that likes Android is less likely to decide to buy an iPad than he is to buy an Android-based tablet, on which is existing Android applications will (generally) run. iOS is much the same, and Microsoft is seeking this across desktops and tablets with Metro (or whatever they're calling it now).
Even within a single "platform" you can have apps that are available on some devices but not others. It's not that hard to find Android apps that are only available on phones, not tablets, and vice versa, or for that matter ones that work on some phones but not others.
This seems like a bad assumption, and a worse one going forward.
And right now, I end up having to find the right device to run that app I want to use instead of using the device that I like with the app that I like...