I agree that it's a noble goal, but I'm curious about places where that sort of logic is actually common enough to justify the overhead of maintaining a cross-platform library.
Most of the applications on my phone are ultimately native UI skins over web services. There might be some logic in there to handle other features, but in most cases it's tied into OS-native features anyway.
I can see cross-platform libraries being useful for games using GL, maybe for media applications that require formats not supported by the OS, and for applications that do heavy image or audio processing – in short, the sort of thing where the value-add is specifically high-performance processing. Maybe that's enough to justify the existence of these tools, though I'd imagine it's almost always a better option to write your library in C in the first place.