The holy trinity of tank, healer, and damage dealer didn't come around because MMO developers decided to make us all codependent. Although a lot of people think that MMO design deliberately created this system of checks and balances, the origin of the holy trinity is far more archaic than that.
In ancient times, before democracy was invented and dinosaurs still roamed the earth, there was this really awesome game called Dungeons and Dragons. Classes in D&D weren't created because they filled a particular role in a group; they were created because they filled a role in the D&D mythos.
Sure, they probably gave clerics and druids the ability to heal because they realized people would need a way to recover lost hit points, but the Fighter, Paladin, and Ranger classes were made tough because the designers thought that they should be strong, tough, and be able to rip up bad guys with their weapons. Clerics weren't just there as healers, they were religious figures with roleplaying depth. It was only natural that they have healing powers, but that wasn't their only role in battle.
Obviously, when you actually played D&D, there was that quintessential need for someone to play the cleric, because, frankly, you needed a healer.
Games have grown a little from the ancient times, and now we aren't so co-dependent on a tank to absorb our damage or a healer to bring the tank back up to full health. But one thing that was sort of lost by that is that same need for someone else to do their job and do it well. When a player doesn't have to have a healer to back them or a tank to cover them, teamwork falters and the need to even play with others at all gets weaker.
This is really evident in Champions Online. You can solo your way to the level cap, and there are even some pretty crazy builds that can solo most 5-man lairs. When you don't need anyone else, the role that others bring to the table is diminished. The harmony that comes with a well-oiled team is reduced when your buddies are just there to kill mobs faster.
Thankfully, all is not lost. The holy trinity is alive and kicking in CO, and even if they aren't needed in day-to-day leveling, there is a select bit of content that does require a team. And, even if you can solo many lairs with a really hacked build, having a team-oriented character to help friends through these rough spots makes them still a rewarding choice to play.
Therakiel's Lair is probably the best example of this. Although Nemesis Confrontation requires a team to start, the team members don't really have to work together at all. Most of NemCon can be done with everyone off in solo mode with little regard for what other players on the team are doing.
Therakiel's Lair requires teamwork. The dungeon has numerous team-oriented puzzles, but on top of that, the boss fights force your team to play smart. You basically need a good tank, and having a support character can quickly turn these much tougher battles from frustrating, multi-attempt wipe fests to fights that take only one or two tries to get the tricks down. If everyone tries to play in solo mode, you will not complete the lair.
Dedicated tanking is probably the most common of the three elements of the holy trinity, since a typical soloing build runs very close to a dedicated tank build. Tanking requires a good defensive passive (Regeneration, Invulnerability, or Defiance), an upgraded blocking skill (of which Energy Shield and Ebon Void are the most effective, but most will work), and some way to hold threat. Most tanks take a couple of taunting powers, augmented with Crippling Challenge or Challenging Strikes advantages as a way to hold enemy attention. Most tanks also pick up active defenses or self heal powers as a way of being less dependent on support characters.
Although virtually all characters in Champions Online can effectively deal damage, a dedicated damage dealer is a frightening thing. Many players decry offensive passives such as Fiery Form or Electric Form, claiming that they are too weak. The reality of this is that while they probably are a little underpowered, the actual problem with them is that they aren't very solo-friendly. When we compare a character built for Avenger Role damage dealing to a character running in Guardian Role with a tank passive, the difference is over 50% more -- so much more that it's actually somewhat difficult for a tank to hold aggro from it.
Dedicated support is a difficult thing to do in CO, just because you need some form of DPS to get you through the main game. Still, there's a lot of benefit from running some form of support passive, such as Aura of Radiant Protection (which doubles as a weaker tank passive) or Seraphim (which doubles as a damage passive). Healing and protection powers such as Psionic Healing, Celestial Conduit, and Mindful Reinforcement act as a huge force multiplier to tanks, and make squishy Avenger Role damage dealers much less fragile.
It's a great thing that the holy trinity isn't needed for the normal leveling content. I love the fact that I can run around in a support build and still take mobs down without having to deviate too much from being a 'healer.' I like that I can make a tank and not have to solo like a snail due to anemic damage.
The presence of the holy trinity in a game isn't cause for alarm -- it means that picking focused roles and teaming aren't obsolete in favor of people playing self-contained tank nukers. It means that people who want to focus on playing a team role and doing well at it have some avenues that they can explore. When 99% of the game is built for solo gaming, it's nice to have at least 1% where having team roles does a lot of good.