Let me start out and say that ESO is NOT like that other popular MMO out there ( I refuse to use it's name. )
In a Dungeon run, the " Holly Trinity"... Tank, Heals and DPS is a part of ESO, but it is not what you may think. The lines that separate these roles are very blurry.
In regards to Heals...
The "Healer" will be able to support the other roles during a dungeon run, but ( A VERY BIG BUT HERE ) A " Healer ", in ESO, is NOT a " FOOL " proof support member. ( emphasis on the Fool part )
Each and every member of a group MUST take their own survivability into account.
-There are no focused heals so you have to be aware of your surroundings.
- If you stand in a ring of fire during a battle with a mob, you will take to much damage for the healer to compensate for.
- If, during a Boss fight, you stand in It's AOE, you will take to much damage for the healer to compensate for.
- If you run out side the healers AOE during a heal, you will not get any heals.
- If a healer comes under attack by mobs, he / she will defend themselves and thusly be less likely to be able to offer any heals for a short time, until they are once again free from attack.
ESO is NOT like that other popular MMO. The Tank cannot just run up to a mob and start swinging away at it and expect the " Healer " to keep them up during the fight.
In ESO, " Healers " are NOT your safety net. Once you understand this small part of the game, you will find that you and your group will survive much longer.