You probably wouldn't unless you disabled URL monikers. If URL monikers is disabled it would be ID vs username. With URL monikers it is moniker vs username. In many cases usernames are exactly the same as monikers.

Okay, so the option is irrelevant unless another option is changed. Doesn't say that in the help text, so to most people it will just appear to not make any discernible difference.

The config option description will be changed slightly.

Thanks :)