Several years ago, I was towing my Jeep behind my Ram 3500 and camper. I got blocked in in a parking lot and had to back up. I made it about 5 feet straight back when the front wheels of the Jeep suddenly slammed over against the left turn stops. This is a normal consequence of the caster of the steering geometry. (The caster is also what keeps the front wheels following in a straight line when going forward.)
It has been said that a person can sit in the driver's seat, with the engine running, and hold the steering wheel straight. It has also been said that this is a good way to get a broken wrist. I don't know if there is truth in either statement, and I don't choose to find out. I only know I tried pushing my Jeep back with the towbar once, it did not work, and I won't try it again. It isn't that difficult to disconnect, move the vehicles, and reconnect.
Dolly manufacturers also recommend against attempting to back up with a vehicle on the dolly.
A car trailer, however, can be backed up anytime, anywhere, even around corners, limited only by the skill of the driver.