Yes, I have a practical need for dest=SUPPRESS.  One of the reasons is basically what you said: to implement things analogous to --help.  In those cases you want to be able to invoke a function (e.g. via type=myfunc) but not store anything.  Or you may need to ignore an argument completely for compatibility, but you have logic which expects all the arguments returned by parse_args() to be known and meaningful.  In any case, I wrote this ticket because I ran into a concrete need that argparse didn't readily support (which almost never happens!).

As for what the usage statement should show: it should behave as if dest=SUPPRESS were not present.  If metavar is specified, use that, otherwise use the option name as if dest were not passed in.  And we already have the behavior that help=SUPPRESS will hide it from the usage entirely, so that should still work (i.e. dest=SUPPRESS, help=SUPPRESS should hide the option both from --help and the result of parse_args()).