Honestly, DRv3 was my try to make a big move away from hit points, while having some sense of what a weapon still does. It saw development pains, controversy, adaptation, and then various degrees of acceptance or adoption. I'd say that the feedback I got is generally consistent between "not needed", "much better than DRv2", "necessary evil" and "really great"... so, this thread seems like the typical spectrum of reactions.
So, all I can really share about it is the context of my worldview on it.
If I say "Anti-Materiel Rifle", most people are going to think about a big rifle that's a threat to a tank. That's the evocative vernacular I was aiming for.
So, in the near-end of the classic Robocop movie, when Murphy rolls his car next to the OCP building and gets the big anti-materiel rifle out to destroy the ED-209 in a single hit, it's supposed to be the natural conclusion that "Oh, this is a Light Anti-Mecha weapon. It killed a mecha in a single hit."
I relate with Andrew in thinking its unnecessary, because a large part of DRv3 is meant to be self-evident. But this is a sci-fi setting where people design stuff, and I think it's helpful to be able to place a gun, call it a "Medium Anti-Personnel weapon" and have a general idea of how well it'll do against most people, and how well it'll do against power armor.
If we put Robocop in as a 'light power armor', having an entire police force mow down at him with their larger caliber guns is something Robocop can endure much better and much longer than anyone else could, but DRv3 informs that you can expect a Medium Anti-Personnel weapon to wear down a defense equal to 'Light Power Armor' eventually.
Is something in the light mecha category because of its size, armor plating, thickness of it - I didn't delve into that in DRv3, because it's not its job. DRv3 is not there to tell you how to design stuff. The actual nitty-gritty of the weapon or armor is up to the submission to sell (i.e.: even sheet-thin zesuaium armor will make small-arm fire behave much differently against a light power armor using that for protection; or effective range of the weapon like Ametheliana alluded to).
So, really, they're more ballparks of expectations... which can be useful because we're a lot of different people, with different mindsets and expectations... and it can be nice to have a baseline. Thankfully, it seems to have lived up to its context as 'in-character common sense' more than the ill-fitting straightjacket previous versions were used for.