Seriously, felipe, this sentence is wrong on so many levels. You're not supposed to kill people in games for the experience points. Experience points are a byproduct of progress, not an objective. Your objective in a game is to finish quests, have fun, and beat the game. Not to horde experience points.
Why does he need to define why I play his game for? If I want to kill everyone around to accumulate XP and then proceed to be powerful enough to just kill the guy who was supposed to beat me up and then throw me into the dungeon area where I would try to escape from, that is a strength of the game, as I see it, not a weakness.
I am not even really concerned with combat XP, or with the ability to profit from killing each and every NPC specifically. But as Josh handles XP in general, it seems you will have very few opportunities to go outside the "level band" the designer has prepared for a certain area. Which seems to only make the game less interesting to me. If you want to remove XP from quests, then fine, go ahead, but allow some way for people to earn them dynamically, rather than only by going through "the proper means" (that is quests).
If people abuse the said dynamic means to earn the XP, or if they don't use it enough and get somewhere with a level way under the designer planned for, then th game can be all the better for it. Specially if the combat system is varied enough so that winning when underlevelled is challenging and resource consuming, instead of impossible. Even better if you can use other systems to accomplish this besides direct combat, such as traps, lures, NPC faction relations and what not.
Better yet would be, I think, if the game didn't plan areas for this or that level exclusively, but rather have threats and challenges of very different difficulty sometimes, and then added means that allow you to escape or avoid dangers too great for you to take right now. So that an underlevelled character in a high level dungeon could still manage to explore it a bit, while even if you are more powerful than normally for an area, some of its threats could still challenge and maybe even outclass you.