Why have attunements?
Attunements are useful when the answer to "Are we ready for X?" is not obvious. For example, you are ready for Gruul when you've beaten High King Maulgar. You're ready for Magtheridon when you've beaten Gruul. You're ready for Scarlet Monastery when you hit level 35. If you can ask the question, and get a one-sentence answer from someone, attunement is not necessary.
However, the question of "Are we ready for Karazhan?" is a lot more complex, and generally involves discussing relative gear levels. That's why the attunement process works. If your raid can do the attunement chain together, especially Black Morass, your raid is ready to try Kara. They may not blow through it in a single night, but you won't be out of your league either.
Similarly, "Are we ready for heroics?" is another good question. Reputation is a decent stand-in for gear level and familiarity with instancing. Heroic attunement isn't quite as good as Karazhan, because it's easy to do an unequal amount of instances, and end up with Exalted rep with one faction, while not having Revered with another. I did this with Sha'tar and Lower City reputations. Reputation is a reasonable proxy for readiness, but it is not perfect.
But outside of those two situations, it is very easy to state readiness in terms of other bosses or character level. And so formal attunements in-game are not necessary. There are already de facto attunements.
One thing I've always found--which is not intuitive from the outside--is that the raiding playerbase is very conservative. They are far more likely to err on the safe side when it comes to trying new content, or content which they think may be beyond their current gear level. Half the time, you don't need to slow them down, they will slow themselves down.
Does Blizzard need to slow down access to content?
Jehu, in a comment to the previous post, says that:
Blizzard is in business to make money, so they have to find a way to keep emptying people's pockets on a monthly basis. They know that players that want to experience and see limited access content will do what it takes to get there. Hence, beyond the challenge of the actual content therein, there also has to be a way to slow down access to that content so it is not experienced too quickly, thereby extending the amount of time a player stays "hooked" on the game and keeps paying a monthly fee.
I disagree with this statement. How many of us finished WoW 1.0 content? I didn't, and I'll venture that 99% of the playerbase didn't either. And how much of that was because of attunements? Given that attunements in WoW 1.0 were trivial, I don't think they had any effect on the inability of players to complete content.
Blizzard doesn't need to slow access down to content. Indeed, they need to increase access to existing content. Blizzard doesn't lose people because of lack of content, they lose people because the content is not accessible. Putting up unnecessary barriers just hurts this situation even more.