This is what I love about parties the most. Defend..Heal..Attack..Buff, It adds to the Dynamic feel of battle something huge. I will agree on leveling up doesn't have to require a person to be in a party to progress. but things like Boss missions,Dungeons,FATE etc. All those things I feel should be grouped in some way shape or form. I mean you can "try to solo" what is meant for grouping, but some things should be grouped, depending on what it is your doing.
As long as there is a balance between solo content and party content, you won't hear a complaint out of me.