I don't like the argument that if you don't practice this particular piece at home, then you're not a passionate developer for some particular platform.

Good devs are good devs, and it's difficult to parse time out to learn each individual platform.

The good devs I know can learn stuff really quickly and the ability to engineer things well is fairly platform independent.

If we were talking about different layers (like say a Systems Developer trying to do UI work) that's a completely different story.