Unnecessary matrices?

Just a quick question: I noticed in some models that a few vertexgroups are assigned to matrices that are following more than one bone. In particular on one model, there was two matrices assigned to the upper arms, another for the chest, and another two for both the chest AND the upper arms (one for the left and one for the right of course).
Since the chest bone is parent for the arm bones, the arm's matrices will always follow the chest anyway; so what's the use for having those "hybrid" matrices?
It just seems useless junk to me.

Blizzard models are full of useless junk.

But the one following both the upper arm AND the chest could be for a few in-betweeenie vertices, to give that extra smoothness... Though that would be totally needless, except for the odd cinematic model or maybe a hero.

You avoid some of that stuff where faces start crossing each other at joints (when you rotate by large angles) if you blend the influences of the bones like that.

Really? I didn't know that. I'll be better check then.