The only way this could work is by adding as few NPCs to the game as possible, but we know how lonely and boring that ends up to be. You don't HAVE to mine obsidian, you dont HAVE to find diamond, you don't HAVE to explore, you dont HAVE to craft special items, you don't even HAVE to fight mobs. The obvious glare with this, is that it will force you to do have to collect EXP to level up and "earn" the ability to do certain actions that you should already know how to do and have the ability to do.TL; DR: Adding Human-like Humans to the Game would take too much effort, and no matter how hard you try, they would either have to be inhuman mutes or drones for labor and activities, and it still wouldn't be human enough. You can do whatever you wish, at any given time, when you want to. It would be an unneeded system that will prove to either restrict gameplay by herding the player into the EXP rush to get enough levels to survive mob attacks or build special items, or it would be an annoyance rather than a grace.It also balances out the group so everyone isn't a godly arcane swordweilding assassin with perfect defense and the ability to revive everyone with one word. This would cause a lot of things to be enforced when implemented due to the difficulty of managing a city that isn't just filled with pigs and cows.Since Minecraft is primarily a single player game with one player, you would be playing a role in a group that doesn't exist or matters. Having a giant city or fort filled with humans that don't do anything when you kill them, or form tribal moblike societies tends to be inhuman and a bit derpy when not worked out well. If it is too advanced, it takes up way more of the game than it needs to, if it isn't the main point of a "survival" game to begin with.
Think about all of the other useless tiers that will be implemented with this system.This also tends to lead to portions of the game being dedicated to a group of villagers doing things that have no importance of the game and are there just for flavor.Why add such a complicated NPC with advanced behaviors and humanism if all it is there for is to be killed or to make a town area more realistic?The great thing about Minecraft is that every item you have has some marginal use.If it is not an ingredient, it is a catalyst for another block (Dyes, Coal, Buttons), or has a use of it's own (Redstone Dust). Since the uses vary by different items, and not everyone wants items for the same reasons (some people want Lapis Lazuli for dye, rather than Lapis Blocks, and others want Diamond for Armor, rather than Tools).Since the implementation of an EXP system would be useless without some benefits and restrictions, the only way to make this beneficial at all is if you do tedious amounts of work in order to increase your EXP to continue your progress towards your next goal. It woul also introduce the most hated of practices, Grinding.