I think there may have been a time where I would've made the argument for higher mobs in crowded areas. However, endgame merit PT spots are the most crowded areas in the game. Still it seems being able to pull mobs that give 70-90 XP consistently gives more XP over time.

I think the biggest reason is that there is still no down time. In a crowded area you often need to pull as soon as a mob pops. If your PT is overcamping and is still resting because the last mob took 5 minutes and all of your MP, you may not be able to pull because your mages will still be low on MP. In this situation it's best to never be caught with your pants down so to speak. If you are always ready to pull and have a good, competitive puller, you will still make more XP/hr.