You're literally responding to @Kyutaru giving you a reason for XP in RPG's. It helps to encourage players to maintain focus on the themes of the game. A little further back, @PhoenixPhyre gave another reason - milestone tracking. Another purpose is to make players feel less-bad about poor outcomes (you fell down a pit and broke your leg, but at least you got XP!). You then acknowledge that XP can be used as a pacing tool.
You can alter the speed at which players earn XP. I played a fair amount of Dungeon World recently and people were leveling up once or twice per session. I'm not sure how fast you need your players to level up, but I would hardly think that is the fault of XP as a game mechanic.