I really don't know why you think this. The 120mm is an insanely powerful unit. It's basically a howitzer with its long range, except it isn't strong against tanks. Play any 2v2 with these mortars and you'll see how powerful they are.
I have played in 2v2s with this doctrine and it is a lot more usefull than in 1v1s 120 mm mortar included but still for 360 mp it leaves a lot to be desired , i am saying the truth , it failed to decrew a pak by scoring direc shots and actually gave the weapon vet and as a result i lost a tank
, in the same game a tiger escaped with a sliver of health from a very well organised ambush of mine despite me throwing at grenades and ramming it with a t34 from behind i had 1 guard squad and a t34/85 hitting it for minutes and still it would not die , i ought to know since i ragequited immediately afterwards MVGame . Anyway most of my games with the other doctrines result in 20 minute stomps ( with me winning ) but when i take GMC despite i always make it to the late game where it supposed to be actually good with a large force it gets really shaky from there