I don't understand the OP at all.
Giving XP for killing monsters does three things that negatively influence gameplay:
1. It encourages grinding. Killing enemies over and over not because you want to, not because it's fun in itself, but because you perceive that doing it will get you closer to where you want to be later on. When you're forced to slog through long parts of a game you don't want to experience in order to get to the "good stuff," that is inherently bad design. In the absolute best case scenario, per-kill XP incentivises grinding, which is bad, and worst case, makes grinding mandatory to progress in the game because the game becomes balanced around grinding out xp, which is very bad.
2. It encourages you to play a certain way even when you're not grinding. Here's a quest to get the goblet from the monster, do you solve it peacefully or violently? Well you get more XP for getting the goblet and killing the monster for his XP instead of, say, completing the quest by making friends with the monster and trading some item that's useless to you for the goblet, so in this case you are effectively punished for thinking creatively to solve a problem because you miss out on that extra goblin killing XP.
3. It makes designing balanced encounters late in a single player game almost impossible, because you have to account for both players who do and who do not grind. How much health do you give the last boss of the game when it can be encountered by a party of level 15 characters all the way up to level 25? Match it to the level 25 party and the level 15's have to grind 10 levels until they're up to snuff. Match it to the level 15 party and the level 25 party cakewalks over the last boss and the player feels like the ending is anti-climactic. At this point you could suggest level scaling the boss, but if you level scale then why have levels at all in the first place?
The same thing happened with Deus Ex: Human Revolution. The most XP you could get from an enemy was the most tedious way to play. Sneak up to the guy, get sneak XP, knock guy out, get knock out XP, after guy is unconscious, kill him to get kill XP. Optimal gameplay was the least fun gameplay. Optimal gameplay shouldn't be punished. That game should have done what Pillars is doing... provided all XP rewards on quest completion which frees you up to actually do the quests however you want.
So in other words, XP per quest only is a fantastic way to distribute XP because it doesn't reward playing one way over another, it simply rewards content completion, which is how it should be.