The attachment's effect is much more significant on the Assault Rifles and LMGs, which benefit from a movement increase much larger than the SMGs and shotguns do. Considering that SMGs and shotguns already have good strafing speed to begin with, this attachment is a rare sight on those weapons. Assault rifles are the overall biggest beneficiary of the Stock's effect, since they are mid-range weapons that are of an average weight but have poor strafe speeds that don't correspond well with other handling traits assault rifles have. While the LMGs stand a lot to gain from the attachment as well, some players can play fine without it when deploying the LMG in a very defensive manner, which some players will consider to be the optimal way to use LMGs.
The Stock returns in Call of Duty: Black Ops III, but the increase in movement speed while aiming is less drastic on assault rifles, sniper rifles, and LMGs than in previous games. All weapons in these categories cannot move fast enough to keep Ghost active even when aiming with the Stock, while SMGs and shotguns cannot do so without the Stock.