I also like to use sports analogies when discussing raiding; it also helps when talking about raids to those unfamiliar with Warcraft and the time commitments.

In sports, you have different levels of play. You have recreational, professional, high school, college, etc. But just because someone plays at a recreational level doesn't mean they're not expected to give 100%, players at each level are expected to give 100% (or more).

As a raider, it doesn't matter if you're in a casual raiding guild, a "hard core" raiding guild, or in a PUG. Your fellow raiders are expecting a certain level of performance out of you and one another, just as athletes expect such from their teammates.

As a healer, you're in one of the key defensive positions in the raid. You do have a specific job to do, and you're usually expected to do so in a specific way - casting certain spells in favor over others, following the tactical guidelines presented by your raid leader and/or main healer (in this case, the raid member assigning healing assignments). There often isn't a large margin of error in raids - and losing a healer in a fire because of "healbot tunnel vision" can easily cause a wipe which is fun for no one.

But at the same time, the healers expect a certain level of performance from the rest of the raid. That the tanks have enough survivability to, well, survive between heals, that the tanks are knowledgeable and capable enough to position the boss for effective dps. That the DPS is pushing out enough damage that the boss is dead before the healers are out of mana, and that the DPS doesn't stand in the fire.

Sure, that means raiding's not for everyone, and there's certainly other activities available for those that choose not to raid. As ultimately it's your decision on what to do for fun in this game.