The title pretty much sums it up, I think it would be good being able to take AI's position in a vehicle as a human player. You often see a stupid bot to take your spawn truck for a ride when you're wanting to backdoor, only to bump it to a few trees and get stuck there. This happens a lot with all the vehicles.

Planned.

We are essentially working on the vehicle commanding logic for for the upcoming 0.84 version. Once this is done, the next step in that direction could be the ability to make a vehicle stop to wait for you if you call for a "taxi" as you probably all experienced it often enough that a vehicle drives away in front of your nose and that can be frustrating.
About you to be able to take over an allied AI vehicle this could be done as well, but for that you'd have to be higher rank than the commander in the vehicle I'd say.

Sounds perfect to me. Given the current rank structure of the game it wouldn't make sense to take vehicles from higher ranked soldiers. To me that's part of the fun: working your way up the chain of command.