It avoids maintaining separate manpages and help messages (thanks to its declarative style), it’s more than a parsing library! I think it leads to better coding standards since you can’t mix parsing with your program’s real work.
I know my user base (me). I find Core.Command's option documentation features quite adequate; I do like to be able to get a reminder of the syntax I’ve specified, because it’s complicated, and Command provides helpful non-man style output for this purpose. I could imagine needing less for some apps, however.