nobody needs a devil's advocate, it is a cheap way of making an argument for the sake of making an argument.

leather does not add distinctiveness to your app, it creates an association between your app and leather that now shares a bond with the 100s of other apps that use leather. the only apps i can think of that have used leather to such an abstract, memorable extreme are paypal (blue leather wtf?) and find my friends, the classic.

mental models have nothing to do with this, unless the average user has a previous experience that associates your product/service/offering with leather. even cowboy boots, even some sort of evocative mood, whatever, that's fine, but if you are using leather because you think you're making it distinct, there are far too many others doing the same.