The advantage of Penal being non-doctrinal, is being able to use them with every Sov armor in existence.
Wasn't flame rifle popular before they got the terminator veterancy?
and I also suppose giving satchel the ability to snare vehicle.
Rifles arrived the field with vet 1/2. the commander gave them an huge advantage in early game + flamer + the ability to counter light vehicles by small arms fire and AT nade. And the point is, that u didnt need to spend fuel for them.
Penals force u to play T1 which can be countered by 222. Penals have no At abilitys and they are weaker than rifles. Also T1 would force you to choose an commander with Guards or Tank hunter Partisans. I mean they arent bad, but there are probably better commanders with shocks/IS2.
Conclusion: No reason to play Penals in current patch. Conscripts and Engineers are doing a better job than Penals.