Kill count has its place, but where it breaks down as a DPS measurement is as follows:
1. Not all NPCs have the same HP - so an AoE effect taking out a group of spiders/bats can help expand upon the kill count, but would not be a measure of anything more than being quick to the AoE punch
2. Quickness in blows - Some builds/styles have a faster ratio of hits, so can squeeze in more hits, this can give advantage by allowing them to get the last hit be it for 6 pts or hundreds of points of damage
Example with a Tank build holding Agro of a group of NPCs while the rest take on a few at a time, the Tank could potentially do the majority of the damage while not getting the kill count. And yes, I know many will state you don't need a tank in DDO, that is not the point, the point is a good tank can still do a fair amount of damage and still not get a kill.
Measuring your characters self worth by the # of Kills next to your name is simply a futile pursuit. I like to measure my success as a character based on how fast we can complete the quest with everyone getting to the end. If that means spending time helping others set up for higher DPS - Such as providing targets that take +50% more damage or helping spend less time healing and more time beating, then I consider it a success.
I was once called out while on my Wizard for not contributing, but it was funny to not only point out that while everything was standing still for him to kill, I also had 3xs the kill they had, and the Cleric in the group still had 3/4 of their SP and we were 2/3 of the way through the quest. Just saying to be careful if you want to use Kill Count as a measure of worth, you may call out the wrong person.