Length matters! Within reason, doubling the length of an antenna increases it radiation resistance by a factor of four, with a similar increase in efficiency. HF mobile antennas are already a compromise, so why would you add insult to injury?
Not all folks can roof mount their antennas, if for no other reason than garage clearance. This is especially true of vans, SUVs, and some trucks. For some reason, no one thinks about mounting their antenna at the FRONT of the vehicle. Not the best for sure, but better than mounting them next to the body of a van.
Remember too, it is the metal UNDER the antenna, not along side, that counts. And a ground strap will not negate this fact!