Tanks can no longer 'creep' when the engines are off. Why? Because you can't move a multi-ton tracked vehicle by popping it in reverse and pushing the gas when the engine is off.
Turning the traverse speed down when the engine is off is along these same lines. I do not know what mechanism every tank used to power their turrets but I know that both the Tiger and Tiger II had a hydraulic pump that was powered by the drive motor. The pressure was then used to power a hydraulic motor that in turn rotated the turret. When the engine was off the turret was manually rotated by hand wheels at a markedly decreased rate. This wasn't all bad; with the engine off you were not making noise and giving away your position. We all call for everyone to shut down their engines in game so that we can hear the enemy coming. This puts them at a huge disadvantage while the powered down tank can still move his turret at the powered traverse rate.
Having the traverse rate changed when the tank has it's engine shut down will cause everyone to make a decision; give away your position and keep your powered turret, or stay concealed and have slower traverse.
For the attackers it will give them some options; either they will be able to hear you running and approach accordingly, or they will have half a chance at using their speed to flank you because of your slower traverse.
I think this is an important issue with the addition of the Tiger II.
It may not affect all tanks. Some models of the Panzer IV had a small gas engine that powered the hydraulics for the turret traverse so it would be reasonable to let the Panzer keep it's powered traverse even when the main engine is shut down.
It is conceivable that some tanks had an electric/hydraulic hybrid where an electric motor ran the pump or maybe through a gear reduction used an electric motor directly allowing the traverse speed to be maintained.
I think this is a pretty simple thing to implement that will lead to a more dynamic ground battle with less sit-and-ambush tactics and more fire-and-maneuver going on.