Luckily as we all have agreed, ESPN has cleared all this up with their nearly perfect QBR rating system. (J/K)
Seriously, I think the flaw in the idea of a new stat is thinking stats can tell the whole story and as long as NFL football is played by human beings that's just not going to be possible. Certainly stats tell a good part of the story but I don't believe stats will ever be perfected to the point where they tell the whole story.
With regard to QBs being statistically "punished" when receivers play "tip drill", given enough attempts it should even out regarding all QBs. Also, what about crappy throws that receivers make great plays on? Would you want another stat for that? No, using stats to make a case makes all kinds of sense, but very very rarely will stats make the entire case. For that we have to use our eyes, minds, and experience in evaluating whatever is in question.