(based on my memory of those times) one of the original reasons for exp being granted on quest completion and not on a per kill bases was (again, based on my memory of discussions about DDO happening in the very early days) was the original devs wanted to encourage different playstyles and avoid the mindless murder-hobo questing of most other games. What matters is completing the quest, not how many things you killed. If you killed a lot, you got a bonus, but if you killed very few, you also got a bonus.
While I'm not completely lost in my rose tinted glasses (there is plenty about the current game that is way better now than it used to be imo), I do miss some of the things that DDO brought to the table back in those days. This (early days stealth play) is one of those things I do miss.