It's kind of a toss up when it comes to levelling in this game. Sure, it's kind of fun playing with your friends and levelling slower but that's part of working together and being all "us vs the world" mentality. But at the same time if you only have so much time to enjoy the entire game PLing is a good way to be able to access all of it. Being level 50 doesn't mean you can't enjoy the quests below 50. It just means the combat parts will be incredibly easy.