I think you should take some appraisal courses. You are so wrong on so many fronts it is not funny. You love to make up scenarios (100+/- sq. ft. differences) to prove your point. Square Footage is used all the time by agents listing and selling homes. They are used to do BPO;s to get the listing and used to advice purchasers of the "correct" price for that home. The problem is the GLA being used is from the tax records. You are also wrong about agents not being able to measure a house. In some states it is required.
The GLA is just ONE item in valuing a home. It is just as easy with your philosophy to attack and or disregard one other single item in valuing a home. That is why there are many factors used in addition to the GLA measurement. You remind me of the hawkers of frozen steaks. "They are just $25.00 per package of 6". Well, how much is that per pound? Well, we don't sell our meat by the pound we sell these fine steaks by the box." Well they are wrong my friend..steak is purchased by the pound..all the way back to the cattle. The same with houses..they are valued and built by the square footage (among other factors!). It does not matter that you can't get an exact square footage as nothing is exact..not even your opinions about floor plans, storage, closet space..talk about inexact and subjective reasonings for demand! You would better spend your time teaching how to measure a house or what is the best ...to get the house appraised (imcluding est. GLA) BEFORE selling. This will save a lot of grief later.