What if you want to offer an option to change user login name later? This is a headache if the rest of your database points to your UID by name.

Same thing if you are making a book database. Sure, in theory the ISBN is a unique book identifier, but what if this somehow doesn't hold (ie, same ISBN but different covers). When you use only the 'natural' identifier you're stuck. Or what if someone typed the ISBN wrong? In a distributed system it can be incredibly difficult to update all the references, for example in users's shopping carts.

For these reasons the canonical DB design advice seems to be to use "meaningless" unique identifiers for objects (doesn't matter if this is autoincrement or UUID etc).