Does the NEC define a "dining room"? 210.52(B)(1)
Regs are designed to account for common usages and provide safe levels of service. Dining rooms often use hotplates or coffee makers that might need higher amperage. A 20 amp circuit requirement attempts to account for that.

There is no use inspector who comes in after your construction to make sure that you follow convention. But houses that do not meet basic expectations are harder to sell, and often result in a compliance problem when they are sold (No closing until we get a new certificate of occupancy!)

That argues for equipping rooms in compliance with what the typical inspector would expect. Most inspectors would think that the room in question was a dining room (since most people would have one in that location). I would wire it as a dining room, but use it as I saw fit. Wiring to a higher standard is never frowned upon, and only costs a bit of wire and a little time, and may avoid headaches down the road.

And, who knows, you may want a dining room and a hotplate someday?

SUPPLEMENT:
After reading Ed Beal's answer, I am inclined to agree so long as there are no countertops in the room in question.