As I understand it, it is possible to hit level cap on quests alone. So in that since you can solo the game, but end game content can't be, dungeons can't be and there are going to be other tough fights you'll need a party for. Plus, when you're in a party each class/job provides a stat bonus to everyone in the party based on their class. So THM will give an increase to INT while Marauders are STR or VIT. So there are benefits to it aside from just achieving things you're unable to do alone.

I'm finally in game as well now. If you want to add me to your friend list or vice versa, I can do it.