While I completely agree with what you say, I feel it must be pointed out that in the use case of using the bundled Play store application on the device, you ha

While I completely agree with what you say, I feel it must be pointed out that in the use case of using the bundled Play store application on the device, you have no way of being certain that it is delivering you the correct APK. It might just as well pull a modified version (there are caveats such as that the device can't already have a valid version of the application setup for this to go unnoticed). Pulling the apk from Play using something like Raccoon would allow you to verify the signature.