How so? You dont have downside of having 6 man crews unless you cap weapons. Regular mortar dies when you have 1 model left, meaning you have 4 spare models to have, unlike other mortar teams which have only 2.
Lets say you was attacked and lost 3 or even 4 models, you still have your mortar alive with original crew which is cheap to reinforce. Other faction would have get mortar de-crewed and would requare to re-cap it.
Even if you cap it with the engi squad, it would still cost you more to reinforce 3 models+1 model on the team weapon, then for soviets to reinforce 4 models of the original crew.
If, in your opinion, there is important down side, in having 6 men crews for your mortar, here is tip for you, do not reinforce you mortars to full and leave them at 4 men.
It is an option after all.
Yeah I sometimes only recrew them without reinforcing. Overall having 6 man is a survivability boost, but like I said, it has a downside because it will bleed you more.
This is a big difference to having a squad of 4, 5, 6 or even 7 (late game cons) soldiers since each of them raises your damage output too and drastically raises their frontline combat time.
Having more men at a mortar (behind the front line) does not raise your damage output. It makes you less prone to get fully wiped but more likely to get hit and thus bleed. I want to exaggerate to make it more clear: If you would have a mortar crewed by 30 soldiers, you would have a pretty much non decrewable weapon which still would fire with only one mortar and would be hit by every single counter shell or rocket because you can't miss 30 soldiers spread around a mortar. So six man obviously bleed you more while raising your survivability in indirect fire fights.
Edit: To make it even more clear: I never said having a 6 man crew is a disadvantage, it just isn't. I only said it has a downside.