I made this comment in TRG forums, when someone brought it up, and I'm going to repeat it here.
In Arma 2, where you were saddled with AI that could neither drive nor shoot, I often wished you could solo a tank, at least as an option... However, in Planetside, there is no manpower shortage, so there is no reason to do away with multi-crew vehicles.
This is one of the tradeoffs for the increased firepower of the larger vehicles/tanks, requiring more people, more teamwork, and it also results in a more effective (depending on who's in it) tank. Driving AND gunning at the same time is a lot for one person to do, that's why it doesn't exist in hardly ANY context in RL. The only instance I can think of is the KA-50... which notably was dropped in favor of the KA-52, which is TWO man with a dedicated pilot and dedicated gunner.
Having a one man tank results in lots of terrible driving and bad aim, or a stationary/slow moving tank with ok aim. Don't believe me? Log in PS1 and watch the first Lightning driver that goes by
Especially in the context of tanks, this is just a bad idea. There's plenty of people to go around to fill all slots in a vehicle.