So in a trinity environment, GW1 for example..

My team includes a Ranger, a Mesmer, a Warrior, and a Monk.

The opposing team includes a Ritualist, a Monk, a Ranger, and a Warrior.

The Rangers' roles are to spread condition pressure and interrupt key skills.

The  Mesmer is in charge of hex pressure, making foes take more risks when activating their skills. They can also be responsible for energy denial and interrupts depending on build.

The Warriors' are in charge of applying direct, consistent pressure to single targets in an attempt to extend the opposing healers into depleting energy. They are in charge of melee, interrupts, knockdowns at pivotal moments, and also preforming these actions on the opposing melee to thwart their roles against his or her own healer (backlining).

The monk is in charge of healing, mitigating, and preventing damage. This may seem black and white, but with a variety of skill options, it becomes very diverse. One skill may block attacks 50% of the time for a short duration, another may turn the next amount of damage an ally receives into the appropriate amount of health instead. Another may even heal after a short delay to counter coordinated spike damage.

The Ritualist role is to support the team in various ways depending on the build. Most viable Ritualists would build a hybrid role of damage, mitigation, and other support mechanics. In this case, the Ritualist acts as spike support and also melee damage mitigation for the Monk.

Keep in mind, roles differ in nature and degree depending on the chosen skill builds. A Warrior may choose to bring Disrupting Chop so that, with the Ranger, they have quite a few options for putting key enemy skills on an extended recharge.

With those explained, i can tell you that in these team compositions, every single role is integral to the next. They all synergize together in meaningful, distinct, and satisfactory instances. They are not homogeneous, they are not generic like a random heal on a GW2 Elementalist every 35 seconds.They most certainly don't oust a player based on his role, because in GW1, you could tell easily how skilled a player was regardless of their role in the team. Either they knew what they were doing or they didn't. This created a very deep, engaging group experience.

Needless to say, i doubt you could describe roles in GW2 with such distinction and true integrity to the team as a whole. That much is obvious when you compare. Because of the depth of these paradigms and the combat mechanics, yes a trinity system, if done well, can promote more group integrity and combat depth than we currently have.

This is why saying that the fate of the team rests on the healer, in most cases, is very false, at least in GW1. If you let a hammer Warrior knock your own monk on his duff because you were unaware, the monk may die. If a Ranger didn't interrupt the Mesmer's Backfire on the Monk, the Monk may die. If the Ritualist get's focused because he isn't paying attention and doesn't apply his weapon spells to effectively mitigate pressure, he will die and your Monk will have more pressure on the rest of the team, killing his energy resources. Every part integral to the whole, deep mechanics, and team reliance. 

GW2 has no such depth. 

When you are in a team, you fail as a team. That is how things work in reality. People have roles that depend on other roles. MMORPGS are supposed to have people playing together in meaningful, deep, engaging ways. They are supposed to have social interaction where people play in groups. There needs to be more reason to be distinct in a group.

People are a macrocosm of this system. We need to delegate roles. We have a desire to be distinct and appreciated for what those roles may be. 

If you don't want to play with other people in a team and accept that if the TEAM fails, it is the whole team's fault because a team fails as a team...then i don't think you are suited to MMORPGs. You play MMORPGs to have meaningful interaction and gameplay experiences with other people, not to solo content and be a jack of all trades, master of none that does the same exact thing regardless of class because the roles aren't mechanically distinct, only distinct in flavor and aesthetic. That is poor design and will not support this game for the long haul.

For these reasons, a trinity system does not always, nor should it ideally place blame on any single player because it is a team-oriented structure. GW2 has an inconsequential team structure that encourages zerg and shallow mechanics. Furthermore, distinct and varied (healing, damage, support, damage soak) builds and roles only serve to deepen the gameplay when implemented correctly as evidenced by the team role breakdown and how each role is integral to the next.

You may agree, you may disagree. I feel i have provided sufficient response as to why this would benefit the game in my opinion. It may not align with the design direction ArenaNet has in mind for GW2 or your personal views, but that does not invalidate what i have said.