Honestly, I don't think there is any need to require a party for Leveling in an MMO. I do however believe there should be a large amount of content that cannot under any circumstances be completed alone. At the very least not without severe difficulty. And I dont mean just endgame content. MMO's should have a very significant amount of Solo and Group content, but I do not believe they should ever overlap. You should not be allowed to Solo group content and there should not be an Easymode setting so you can. Likewise, you should not be able to group your way through Solo content. They should both be equally difficult and cater to their respective crowds.

Sometimes you want to play together. Sometimes you want to play alone. But you always want to get more powerful. How you do it should be up to you. But they should not ruin everyone else's experience just so you can play the way you want.