I saw a blog post on this subject on Snarkcraft a while back called "Healing Myself Out of a Job," and found it very interesting.
Let's explore this.
In progression raiding, the goal of the raid is to down bosses as quickly and painlessly as possible, moving through the instance until all encounters are down and solid.
There are 3 basic roles in a raid: tank, DPS, and heals. We don't need to get down to the specifics of what kind of DPS, or pullers/CCers, etc., here because it's irrelevant. What is interesting, however, is how the individuals in these roles are affected as the raid gets better and better.
The tanks: Encounters generally call for a set number of tanks. For example, when my guild raids 25-man Ulduar, we bring 3 tanks. Sometimes we only need 1, sometimes only 2, and each encounter is tanked by the player that the raid leader deems most suited to it. In short, the number of tanks you bring generally stays the same as the raid gets better. You always need tanks. As individual players get better, they may be subbed in for other players who are not as effective or have less than optimal gear.
The DPS: When raid assignments go out, the amount of DPS is usually set at a certain number. High DPS is important, because it is required to bring bosses down. Simple enough, right? So as the raid gets better at a certain encounter, and the DPS is higher, what usually happens? The number of DPS brought to the raid either remains the same or increases to aid in a faster kill time.
I'm sure you're wondering where I'm going with this, so bear with me for a second...
The healers: When raids first begin an instance, such as 25 Ulduar, they bring x number of healers. For our purposes, we'll say 6. Some encounters at the beginning simply require that many heals to make sure things go smoothly as the raid learns the encounters.
However, here is where healing differs from tanking and DPS. As DPS goes up, tanks get better at encounters, and the healing begins to handle the damage more smoothly, what happens?
You bring less healers.
The healing load lightens as everyone improves, and so those 5th and 6th healers become overkill that could be replaced with another DPS to speed up kill time.
So essentially, healers heal themselves or their comrades out of a job as they get better.
I feel like that might be why the healing "department," if you will, of a raid can be so competitive and meter crazy. We know that the number of healers needed for an encounter will go down as the raid as a whole improves, even if we don't actively think about it. We know that the best healers will be the ones to stay in that role, barring some kind of composition requirement (which Blizzard seems to be actively trying to remove anyway).
Sometimes Numbah 5 and Numbah 6 have DPS offspecs that they can switch to and remain in the raid. However, not all guilds work this way, and many would prefer to bring their best healers and their best DPS over their best healers, their best DPS, and the extra healers that are not needed in healing anymore as offspec DPS.
And you're kinda screwed if your offspec is PVP.
It's a very intriguing process, at least to me.
If you're a tank, you know there are set slots for encounters, and your goal is to get better to be one of those set numbers.
If you're DPS, you want high DPS and good CC. Getting better helps the raid, and as the raid improves, you're likely to add TO your DPS lineup to speed up kills.
If you're a healer, you very well may not be needed once the raid gets better. Your goal is to be an amazing and efficient healer so that you can remain in the raid once spots start disappearing.
Thanks so much to Ambient for linking me the OP! I could not, for the life of me, seem to remember where I saw it.