Like said picaron : they are very cheap for the main advantage it give you : to not let any urban region of the country to the enemy without fighting. Even if you lose all this fights, the enemy will lose some troops and after all they will lose time to recover cohesion and clean the areas... It is probably a nice tool for the Reds which mostly need just time to get stronger and stronger ...
Besides this, the few ones you could train automatically with leader who have the ability to do it are a way to build good units for a very low cost.
Finally, (as usual i could say :bonk
, don't care of the current 'Brutal occupier' ability which is among the ones which are not working in the vanilla version. The next patch/upgrade will give the Milicias a more clear 'painfull aby' called 'Insurrectional troops' : All the enemy units with a (also new) 'military police' aby (like cavalry cossacks or Cheka/VOKHR units), will fight them with a good bonus in combat. ...Just because there is always some professional guys to waste the party
.