I don't think it's particularly wise to try to determine the effectiveness of a player based solely on scoreboard stats (especially sans objectives). It's sort of like trying to recreate a baseball game based on the box score. Sure, it will give you an idea of what happened, but you won't have the full picture (or the slo-mo replay of that controversial call, etc.).
High Damage + Low Killing Blows = a) Someone attacking inefficiently - like somebody with a pocket healer? Or maybe b) someone who is beating the pants off of everyone, only to have the kill sniped by another player who one-shots the nearly-dead enemy?
Low Damage + High Killing Blows = a) Somebody who is "stealing" kills - for example, using high-damage instants to finish off an enemy that was going to die to a teammate anyway? Or maybe b) selecting prime targets, picking off the almost dead before they have a chance to - or perhaps while trying to - eat/bandage/heal?
How do you know the difference between outright kill-stealing and an assist unless you saw it? I've been on both sides of the equation. When I'm fighting somebody and I'm at or near full health, haven't blown any cooldowns, and my opponent is on his last legs, I get upset when a rogue ambushes to finish him off or a DK deathgrips him 30 yards away from me to land the KB. I put in the effort and I deserved the stats as a reward. On the other hand, I've also been in close battles, where both my toon and my opponent are struggling to stay up. In those instances I welcome assistance and thank the guy who saved my butt with his timely heal/kill.
I've also been in the situation where I didn't get help, emerged victorious from the scuffle, and then was insta-gibbed by an ambush before I could get combat-ready again. Although I admit my first thought is always "F'ing rogues" (albeit in less forum-friendly language), it's always followed by understanding and respect. I understand why I was a prime target for the rogue/kitty to finish off and I would do the same in that situation. If I can't get to the fight fast enough to save my teammate, I can at least make sure the guy who killed him gets a trip to the graveyard, too. You don't want somebody who wins the close battles to get a second wind and find someone else to maul. You take a page out of Barney Fife's book and nip it in the bud.
The thing is, all of those scenarios play out the same way on the scoreboard (low damage, high kills). That, to me, is why you need to worry less about scoreboards and more about actual play. It's a team game, so teamwork is the important part. If you're helping people that actually need it; cool, you're doing your part. If you're sniping kills to try to pad your stats rather than rendering help where it's needed most, you're not really helping anybody.
Edited by Tempestuous on 10/4/2012 1:36 AM PDT