I tend to write this on all other RPG forums, so why not here as well?
According to research, especially overjustification effect and CET, getting a tangible reward for a task does nothing or can even decrease the enjoyment of doing the task. The only exceptions to this is if the reward is social (like “building on someone’s idea”, “giving praise/like” or otherwise acknowledge someone), if it’s a competition (like, “combat” or in sports), or if the task itself is really boring.
XP is a clear pointer though, but it would be an equally good pointer to say “Roll the dice to overcome an obstacle, but you need to involve one of your personality traits when you narrate the outcome”. Instead of luring with cookies, you force a behavior while the players tries to reach their goal. Reward driven design is when the game designer tries to reach their goal through bribing the players, but interaction driven design is when the game designer’s goal is along the way of the players’.
Anyways, I’m in the camp where getting a tangible reward (like XP) decreases my enjoyment of doing a task, but because everybody doesn’t feel this way, it’s hard to talk about this. I personally prefer doing a task because the task itself is fun. More on that in the CET link above (which people that click on will enjoy because the task itself - learning about it - is fun for those people - also: a video).