This would prevent some awkward moments getting in and out of tanks, like when both driver and gunner get out to repair at the same time.
Edit: To clarify:
- This question is for PC.
- I already know that once in the vehicle you can move around between seats with the function keys. This is for avoiding those few seconds lost when both driver and gunner go out to repair and get back in in the wrong order. In those cases, one person needs to get out in order to let the other switch seats, which is an annoying, and a semi-frequent occurrence if you're in the habit of forming stable Driver/Gunner teams. Basically, if I know I want to be in the gunner seat, then why force me to get in the driver seat first?