If the area meets the ANSI standard for calculated GLA, it's GLA. The USE of the room is not a part of the analysis. You are mixing in concepts of functional utility, contribution, overimprovement, and superadequacy into the determination if whether or not it is GLA. How you handle these MARKET DRIVEN forces on the area is of no relevance to whether or not it is considered GLA. The value you place on the additional area and possible adjustments for it are market driven; the determination of whether or not it's GLA is purely based on physical features listed in the ANSI standards for GLA calculation, which are used by appraisers for consistency throughout the profession. It's pretty black and white.
I suggest you obtain a copy of the American National Standard for Single Family Residential Buildings Square Footage-Method of Calculating.