I have trouble recognizing a 6 man crew as being beneficial, other than anecdotally. Axis infantry drop models the fastest in the game and the anti tank gun crew don't put up a fight at all. Arty is still gonna 1 shot all 6 dudes like any other at gun.
Besides, the crew can't move faster than a snail (what is it like half speed?) so once the gunner or the loader dies, they just get stuck in a death loop and never fire again. Might be all at guns and support weapons doing this, but 6 men is just more experience, more popcap consumed, and more lols.
So first having 66% more hit points before being decrewed is a durability upgrade however you slice it. The gunner and loader are not always the ones targeted first by small arms and that means more time for you to back up into safer territory and/or bring reinforcements up and/or get your machine gun facing the correct direction to cover your AT gun.
4 men firing back is also an advantage when you crew it with soviet infantry. With 4 conscripts, penals or combat engineers able to fire back, the zis can contribute a not insubstantial amount of supporting firepower to a given engagement.
Next while lmg obers have one of the highest dps at long range in the game(if not the highest), they do no represent the majority of axis squads. If you compare allied squads to their rough axis counterparts you should notice that the dps really doesn't favor axis overall.
Arty doesn't always one shot weapons teams unless you are talking a about stupa dive bombs or railway artillery. A single hit or two from a mortar is not enough to kill a full hp zis crew unless they are standing on top of each other.
The zis does not actually take up more popcap than other at guns. It takes up 7 just like any other. Similarly the raketenwerfer also takes up 7 despite having 5 crew.
I can tell you right now that as soviets, my stolen PAK 40s are a lot easier to keep alive then when I use them as ostheer because of aforementioned reasons.
Some situations you just are going to inevitably lose your weapon teams but a good player minimizes those and in all other circumstances the extra crew is absolutely an advantage.