is USES=ncurses useless?

[Vick Khera]

I've observed (and filed PR 206471
<https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=206471>) that the
USES=ncurses flag in a port does not result in the port actually linking
with the ncurses library provided by the port.

Does anyone else notice this? Specifically two ports I've encountered that
have this issue are tmux and tty-clock. They both specify the ncurses from
ports, yet link to base system ncurses library. They register dependencies
against the ncurses port, too, which seems to be the only accomplishment of
this setting.

Full gory details are in the PR linked above. I'd appreciate any comments.