I suppose you could rephrase this question as "When can design supersede User Experience?"
I've just designed a Nav bar that goes against everything I stand for as a UX Professional, yet I feel I can justify forgoing best practices here given the websites audience.
Here's a quick snapshot to give you an idea of what I mean:
As you can see, each nav item is bunched up against the next. Making it a tad difficult to read. The target audience of the site however, is the young professional male, typically well versed in the ways of the internet and technology. And this gives me the confidence that the user will quickly identify this function and use it appropriately.
So does this justify a design that ignores best practices of UX?
When can it ever?