Whilst I agree that cross-platform is a nice goal, I'd much rather developers wrote what they want to, and are more comfortable with first and foremost without having to think about multiple platforms or 'trendy' languages, etc.
I want developers to write the things they want, and I'll be happy if they share them, rather than writing things they think everyone else wants and not having the same passion.
Some people will want to write multi-platform something and have the passion for that, which is great, but I wouldn't ever want to enforce that on any one.
If you ever want to see a developer cave, just look at Linux (and the oft-used 'Windows people can build it themselves if they can work it out')
Disclaimer: I use all OSes and like them all in different ways. However, when I write software for myself, there's little chance I'm caring about who else will run it. It's coding for pleasure, not money.
Edit: I should also mention how utterly AWESOME Dwarf Fortress is.