Developers are users too. If users cared enough not to use cross-platform software because it didn’t look native, then it wouldn’t get built.
Count one here. The main thing for me is: does it solve my problem? Not: is it native/does it look unique, etc.
And frankly, I think you would be pretty annoyed to try to use software that's designed to look and work like Windows 8's half-baked Metro UI if you're on Mac or Linux. What you said comes from a place of assuming it's a good baseline.