Because he'll be the last one alive to give you your soul gems after a Zerg melts your faces off.
For what it's worth I've been in multiple battles this weekend where I survived for more than 30 minutes near the front lines or on top of a keep wall. I'm able to continuously heal everyone around me at least 3 to 4 times before I'm out of magic. And when I'm out of magic I'll use my stamina based powers to attack things, or just sneak for 30 seconds until my magicka comes back.
I will never understand why so few view their own survivability as important aspects of being a healer.
Lastly, if you cannot at one time or another perform every aspect of the trinity yourself you will struggle mightily in TESO, especially if you solo, and doubly so if you play PVP.