If they don't even keep track of what they graded, they don't deserve to be in the registry.

ANY grading company that doesn't maintain a census, either through subscription or free to the public, shouldn't be in the registry.

Include other companies, and you lose the ability to track finest certified, or pops, or any of the other good things that a registry provides. Keep the scoring relative to the known population/grade.

Adding other TPGs will skew individual collection scores upward, unless the scoring system is recalculated downward to account for the additional top pops that surface. Score inflation will have the same effect as grade inflation - making really nice coins more expensive for everyone. $.02