Not so iron, if you have many platforms to manage. Usually it doesn't worth it to manage tons of separate toolkits for each platform, if there is no common abstraction at all. It's just too costly.

If your app looking like shit will cost you significant sales -- as is the case for most productivity and design apps -- then yes, you have to port the GUI bits to each and every toolkit you're using.

Firefox, LibreOffice, VirtualBox and etc. If you don't know how to make good GUI using generic toolkits it doesn't mean it's not possible.

Anyway, if you buy application for its "looks" - there is something seriously wrong already. It should look good, no doubt, but it should be functional first.

>as is the case for most productivity and design apps

Completely the opposite. They tend to provide completely custom, not native looking UIs, and therefore using cross platform toolkits for them only makes more sense.

And I wouldn't describe LibreOffice as "not looking (and running) like shit". Again, nobody on Mac actually uses it, and Windows has the real Office.