Isn't the concept of the trinity simply a necessity to overcome content more than it is a design decision?
Unless I'm completely mistaken there is still going to be someone who takes the hits of the main enemy or at the very least maintains the focus of the enemy to keep them from slaughtering everyone else.
There is still going to be one or more people involved in keep that person and the rest of the group alive.
Finally there will be DPS done to the enemy to kill it.
That's the trinity in a nutshell. In traditional MMO's there is a Tank, Healer, and DPS to fill those roles. Each of those people are focused on their single task. Now in reality we know that each of those people CAN do more than just their one task.
Restro Priest can cast damage spells, tanks can use DPS equipment, and healers can self-heal to tank some times. The point is none of those things are the ideal job for those given classes and specs. In fact many of these classes do have the ability to do 1, 2 or all 3 of these roles when they solo, but in a raid environment its about maximizing your role in the group.
Now in a non-trinity situation maybe it isn't 1 person that is always healing, but you will have someone who's responsibility it is to keep your group alive and someone to do DPS, ect. So why would GW2 be any different in the sense that they will want particular spec/skill combinations to fulfill the role you are bringing to the group. Even if you hybridized the roles such that one person cannot do all the healing for the group you will still have a set of people responsible for filling that role.
So instead of doing heal heal heal heal during the fight you'll do heal, dps, tank, heal, dps, tank...except it will require an even higher level of skill because now instead of 1 person keeping track of 1 thing you have everything trying to keep try of everything.
Tell me I'm wrong...because that's the way it looks to me...