Well, the Guard, stock, has a light BCG (no weights) and a light (3.0 oz) buffer. As far as I can tell, this works with most of the off-the-shelf ammo, which is the goal. I don't think the goal is to "NOT to have a giant hunk of steel slamming back and forth". And the RDB takes a lot of that away anyway.
Now when you are "tuning" a PCC, it is good to have choices. Many have gone to a hydraulic buffer, a different buffer spring and/or different ammo. The weight kit is just an extra option in tuning. You are looking at total weight of the moving mass, BCG + buffer, which is usually less that what blow back PCCs end up with anyway. Some still recommend a 9mm BCG (which has about 3 and a half oz of weight pinned in) and a 6 to 9 oz buffer.