In my view, “cis” does not denote someone who does not suffer from GID. It denotes someone who does not suffer from (or must manage possibility of suffering from) transphobia on a regular basis, except perhaps in rare, exceptional circumstances.
But then butch lesbians, effeminate men, anyone consistently misgendered because of their haircut/clothing/behavior/etc. would not be “cis”, whether or not they were actually transsexual, therefore rendering the term useless?
How does the presence of borderline cases or grey areas make terms that describe institutional oppression and privilege (whether it is “cis” or “white” or “able-bodied” or whatever) “useless”?