It depends. I don't think Eternity is intending to be an open-world kind of game like the first Baldur's Gate, where 90% of gameplay consisted of wandering around through wilderness and caves slaughtering wildlife. In such a game per-kill XP makes sense, or at least "per dungeon" or "per area" or whatever, but generally speaking Obsidian games are more structured than that.
Fantastic example of XP in an open world game: Just Cause 2 and Chaos. Chaos is basically the way you gain progress through the game. It's your currency and it's used to unlock new missions (side and main). You also get it for engaging with the game's primary mechanics: shooting shit, blowing shit up, etc. The Chaos curve is set up so that you get the same Chaos for basically everything regardless of where you are in the game; only the required thresholds for progress continue to increase. By virtue of the GTA-style enemy alertness system, the player is in direct control of his/her challenge level for most of the game. It's all one large, entirely mechanics-driven progression system, and it works phenomenally.
Now tell me, does this system make any sort of sense in a game built around hand-designed scenarios instead of open-world anarchy? Project Eternity is not a game built around abstraction and around perfectly coherent, balanced mechanics and systems, at least not to our knowledge at this time. Furthermore, by having detailed NPC dialogue trees, it has to be able to have enough limits to properly recognize player actions, motives and so on; you can't mesh two completely different levels of abstraction across narrative and mechanics, it just does not typically lead to effective results, and can come across as very dissonant for the player.
See Skyrim for a very bad example of this - quest rewards are typically player level x 100 gold or similar, but this leads to the player completing simplistic fetch quest and earning thousands of gold from rag-wearing merchants selling leeks. Sure, you can make 5, 10, or 20 "quest reward formulas" depending on the quest. At a certain point it just makes more sense to do everything by hand, though.