[Imp]

The simple answer is if probably if someone abuses it dont play them again., if they feilded loads of size zero units or other similar silly tactics you probably wouldnt.

Picking nits & sowing discord.

Double Mgs should be compared to their conterpart, double mortars probably 80mm the closest.
MG probably costs about 2/3rds 3/4s the mortars price.
Similar rates of fire Mortar probably has the edge on kill & range. It can fire anywhere within range with planing rather than restricted Z fire hexes. Can fire smoke (thats worth a lot) & is more effective in cover terrain as bigger warhead.
Its slower & has less ammo.

I fire mortars/arty with one goal only realy & thats suppresion any damage caused is a bonus.
MGs I fire generaly to damage but preferably at a target with an adjacent one, covering open ground very effective or vs that pesky ATG.
That said if I Z fire anything I look to my MGs first but only if needed as I think the arty missed or whatever.
What I am trying to say is most players I have come across dont Z fire them every turn they use z fire sensibly. That said in a flexible system like this there will always be those that abuse it but most people use the tools correctly.