Two excellent points DAR. Re the AeroCar, I would guess that Molt Taylor probably thought it would drive more than fly. Furthermore, remember that it is front-wheel drive, and has to tow the empennage/wings/tail/prop unit behind it. All of these points may have contributed to him making it with four wheels.
I certainly had a memorable couple of days flying and driving it, and would love to sample the Terrafugia.