My biggest problem with this is that we'd be telling new players that the Locusts in their starter boxes (and soon, the Stingers in their Command Lance packs) are doing absolutely nothing with 10% of their mass and have a maximum damage potential of 5 until they buy a big and relatively complicated rulebook and start using infantry.
To be fair, Catalyst just dropped Clan Invasion with Battle Armor, which I do believe Mech MGs should deal damage to. Personally I would take one look at the MG variant and move on over to whatever the other one was in the AGOAC.
I would probably classify the A-10 Warhog's gun as an AC-2 or AC-5 personally, especially when looking at the range.
As for the Phoenix Hawk, if it takes an alpha strike, with MGs as the finishing touch, to force a PSR then it probably isn't a mech designed for forcing PSRs (IMO), especially since you need to be within 3 hexes to even hit with the MGs. That is very dangerous territory for many light and medium mechs.
I will grant that I am coming from a Total Warfare mindset, so it does make some units rather 'irrelevant' until infantry are introduced, however there are plenty of variants of those mechs that don't utilize the machine guns; the Stingers first variant I see on Sarna is the 3G, which has another medium laser. Those variants were probably made for the same reasons I stated, both in-universe and IRL.
I just see Bergan Industries supplier quality in a conversation with a buyer; "we didn't design them to fight a Thug. It has machine guns! Fine, you want something to fight a Thug with? I'll tell our engineers to strap SRMs on it. No, I'm not covering this under warranty. Owner's Manual specifically states not to fight Thugs with this mech."