Security regiments don't help that much; you put them in front, they retreat, sometimes rout or shatter, against a hasty attack.
Yes, they retreat, but closing on them (or going around them) costs MP, attacking them costs MP, and in the first few weeks the Soviet player is desperate for map counters to delay the German advance.
The manpower addition of disbanding security units is outweighed in my opinion by giving the Germans more room to run free.
I just got to December in my current PBEM and have over 700K in manpower reserves.
I'm sorta pragmatic in that I like to use what works even if it seems like a gamey tactic.
It's not gamey, its realistic. When a unit starts taking fire they don't know if its a couple of stragglers, or a whole division they're bumping up against (S2 doesn't know everything).
Stopping, deploying, reconning, and assaulting all eat precious time compared to Sunday driving through the countryside.