I never run my Soviets without mortars, always run a medium and a heavy at Regular status.
They keep static teams (Machine guns, Artillery etc) on the move and unable to fire whenever they move so even if you don't hit anything the opponent is constantly aware of the threat that comes from it.
Maybe they won't move them until you have to get 4+, but it's constantly looming over them that if it hits it'll be devastating!
Plus, I've had amazing rolls with my mortars. Destroyed a Tiger (I think?) with one. Superficial damage, set on fire and he failed the morale test. Not bad for a 50 point weapon
I'd keep them in the list mate, definitely.