I would've thought that the argument for allowing it in private servers is that "Hey, this is our space, we're not being offensive, like it or leave it". This would then apply to clans - nobody's forcing anyone to read threads in clans that they aren't even apart of.

In the end, it boils down to a value judgment. Should be people be free to be non-offensive to each other (yet still using 'nigger') in their private spaces, or should someone's right to be offended trump that? Obviously we all have different opinions on this.