"Runabout" by definition:
1 : one who wanders about : stray
2 : a light usually open wagon, car, or motorboat
#1 May define a Pinto that has bad alignment. Given that Ford ran a commercial with huge fans blowing a crosswind as a Pinto drove by (proving stability) I doubt it is referring to that kind of wandering.
#2 "Light" may accurately refer to a Pinto. "Open" may apply to the hatch after a rear impact.
All kidding aside my recollection was that "Runabout" referred to the fact that a Pinto was well suited to runabout town and accomplish many tasks. The features were easy on gas, small (compared to cars of the day) for easy parking, practical for loading with the rear hatch.
Since Mercury was the upscale division of Ford perhaps (I'm just guessing here) they attempted to separate the Bobcat from the Pinto by not using the term runabout. Then people became confused (the hatchback concept was rather new at the time) and they later applied the term to the Bobcat to "carryover" the concept. Or..., the reverse may have occurred. If "Runabout" was applied to the Bobcat they might have eventually elected to distinguish it from the Pinto by dropping the term. Given that your post did not state the years "Runabout" was applied to the Bobcat it could be either - or none since I'm just speculating (but I think it is a good guess).
Tom