The rockets and shells do have the knockback going for them. The Plasma Pistol still outdamages the rocket launcher and double shotgun with maxed SoaG, and it still has the -60% firing time going for it (how many extra shots would that be?). But without any points in SoaG, you get 2-12 damage before considering any other abilities you may have, which even the chaingun can outstrip handily (and with more plentiful ammo that doesn't compete with the BFG, to boot).
It could very well be the case that someone who sinks points into SoaG might never find the Plasma Pistol in the first place, be it through RNG fickleness or a shotgun guy setting off an adjacent barrel because. For me, at least, unless I knew for a FACT that I could get my hands on one of them (or if I was doing a pistol-only challenge), I'd be better off investing my skill points in other skills.