I don't use it, but I suspect en-5 was originally supposed to be used to
denote those who write English professionally, such as journalists, writers,
and editors. My own profession would probably qualify. However, I only use
the native English one to point out that, yes, I know English as a native,
but I find it really unnecessary from a Babel point of view to point out
that I can write professionally (especially when my quality of edits on
Wikipedia is often subpar).
I don't really see a need for en-5, but I don't begrudge those folks who
want to use it.