Here's a few quotes which I'll present without commentary. From the section "Let's Talk Psychographics":
Wizards R&D certainly believes that there are differences in player psychology that manifest themselves in role choice. In other words, some players are naturally drawn to play a leader, others a defender, and so on. I'm pretty sure that the Blizzard devs believe the same thing. And certainly some players are convinced that "you know, healing is really what I'm good at."
From the section "Set the Wayback Machine for 1982":
The other characters start standing in front of the magic user...and thus the traditional front-rank/back-rank tactic is born. More interestingly, the other characters start doing everything they can (attacks, verbal insults, silly/weird antics) to keep the monsters from focusing their attention on the magic user...and thus aggro management is born. If magic users were as durable as, say, the D&D thieves, I don't think either behavior would have become so pronounced.
From the section "Whither Healers?":
The simulationist in Gary followed a very reasonable line of thinking: If you get stabbed nearly to death, it should take you days or weeks to recover... But as anyone who has run a long-term campaign knows, long recuperation times can be hell on the ongoing narrative... It's worse if some players need to recuperate, but others don't; that's a recipe for splitting the party. And those long recuperation times wreak havoc with any sort of time deadline before the Great Evil Event happens. As a DM, you want that tool in your toolbox.