I think Luke originally had his own car (the one Cooter borrowed) but it is never shown (perhaps Cooter never returned it). As Uncle Jesse said, "Next time you loan your car to a fool like that, just run it off a cliff and save gas!" Then it's explained that Bo and Luke co-owned the General. I'm more prone to believe they wanted the Jeep for the 4WD off roading type stuff. Also, they were able to give her Jeep a name. Besides, it was just too funny to see Daisy chasing Bo and Luke with a frying pan after they drove her car off a cliff!