That's a very good point, but I'd say that mostly the interface philosophies co-exist reasonably well, even on one device. Because apps tend to be full screen

That's a very good point, but I'd say that mostly the interface philosophies co-exist reasonably well, even on one device. Because apps tend to be full screen it's not such a big deal to have them behave a little differently. The hard buttons tend to have predictable behavior and important stuff like 'how does suspend/multitasking work?' is the same on all devices. It's also quite nice being able to use good apps designed for another look-and-feel anyway and decide yourself if an out-of-place interface if worth it. For example people use iTunes on Windows even though it has it's own UI toolkit, because overall it is worth it.