Most these battles take place in the front lines, Partisans mostly operated well behind enemy lines where they didn't have to deal with the mainline combat units that they were ill equipped to deal with. In the campaigns I usually use my recon forces to slip behind enemy lines to search out and destroy supply units, mortar batteries,wire linemen,arty spotters and mechanics teams.
I am new to these games but I am not new to the subject of WWII, judging from what I've seen so far historical accuracy is something that is extremely important to the devs. It is one of the reasons why I love this game.
Partisans played a major role in the war across Europe and particularly in the East, as mentioned in the OP partisans often fought alongside the Russian army in conventional battles they also fought several massive conventional battles without the Russian military.
I agree about using recon units to attack weaker isolated enemy units during main battles and that is what I try to do to, however I was suggesting the addition of partisans to conduct an entirely different facet of war, guerrilla warfare. You could position small groups of partisans in places the army wouldn't be able to get to before a battle.
Partisans could be factored into the campaign mode too. Perhaps it would be possible to select areas for intelligence gathering or request particular strategically important targets to be attacked prior to a main assault or coordinate certain 'events' to occur during the main assault.
For example - you are planning an assault against German forces positioned close to a river crossing, you receive intel that indicates there are forces on both sides of the river so you could ask local partisans to sabotage a bridge. Now, do you take the bridge before or during the battle? there are risks and advantages to both depending on success and failure of the partisans.
Success could mean the Germans south of the river will be cut off and without assistance, failure could mean the Germans will be more aware of an approaching attack and perhaps have better prepared defences in place, which would suggest waiting might be the best option, it opens many possibilities.
And all this really happened, it is well documented, there is lots of information out there including eye witness accounts, especially since many files have been made available to the public from the Russian archives. If this could be added in a way that respects the games historical integrity I believe it would really add something new, interesting and authentic to this great little game.