Having the leveling system as a part of Vanilla Monecraft is more than enough, as of now it can be beneficial by strip mining as you find all these different ores that give you XP. Because the mob spawning is ascending, you can also get more than enough XP from making a mob grinder.
TL;DR McMMO is all about levels. Vanilla Minecraft already has that via the means of ores, smelting, fighting mobs, etc.