I have never found irregulars to be worth it. They have a small squad size, are quite expensive, and you never know exactly what weapons they will have.
After playing quite a bit with partisans on the free weekend I am also struggling to understand how to use them. They are hard to integrate into an army because of their low health even after upgrades, and often end up standing around my HQ all game. They also take out a lot of mp when you start to call them in, and leave you with effectively no units. The commander is not bad per se, I just don't know how to use it effectively.
I use them also during the free weekend almost exclusively on Semois. Harass, harass, swarm your enemy.
Best situation for using them was when i heard from the Radio intercept that they deploys support weapons. Early game, any MG shouldnt be a problem.
Mid to late game they are either pushing behind Cons or just harassing enemy territory. If alone, inmedialy retreat.
Also, people should remember to deploy mines with them.