I think people should use what they are more comfortable with. If someone's more comfortable with XNA, he should use XNA, if he prefers ENIGMA, he should use ENIGMA. Cross-platform compatibility is always good, though, but if you just can't manage to achieve something using a language or tool and can do it much easier in another...
In the end, maybe you can release the source code and someone can try to port it to other platforms if he wants.
Also let's try to not start software/language wars. Not that there are much of them right now, but I see where this can go.
