Cast Times

What is a cast time?

Cast time is the time needed to cast a spell before it takes effect. Essentially, it is time that you "spend" devoted entirely and utterly to whatever ability it is that you are using.
Let's take, for example, fireball. It comes with a base cast time of 3.5 seconds. This means that you "spend" 3.5 seconds standing there, doing absolutely nothing except fireball. If you want to do something, like run so you don't die, you have to stop the fireball. You have to stop casting, which means you wasted time, and move, which spends more time on moving, thus taking away from time that could be better spent casting a spell, and therefore being useful.
Every time you are forced to move, and do nothing, that is time spent where you are useless.

Having instant cast abilities/spells counteracts this, by letting you do stuff while moving.

Let's look at this from a PvE point of view first off. We're going to deal with a fire mage for this.

Big Angry Boss Mob is engaged. Every 20 seconds, he hits every ranged DPS with a ground-targeted spell. Therefore, every 20 seconds, the ranged DPS needs to move, or get killed. At each of these intervals, our mage will be moving for 3 seconds to escape painful death.
Let's say it takes 5 minutes to kill Big Angry Boss Mob.

If our fire mage was allowed to do non-stop DPS for that entire time, our mage would put out 1449 DPS, giving us a total damage dealt of ~434700.

But now let's consider the movement thing. We have the first 20 seconds unfettered, but after that we need to move. After the first 20, we're going to be moving for 3 seconds, and only able to cast for 17 of those seconds.

This gives us 4 minutes 40 seconds of interrupted casting. Working out to 14 cycles of movement, this means that we are not casting for 42 seconds.

Before, we had 5 solid minutes of DPS time. Now, we don't. Now, we have 4 minutes and 18 seconds of DPS time.
In this amount of time, we only deal out ~373800, working out to be 1246 DPS.

So you see what happened? Our damage and DPS gets slammed because we have to move, therefore spending time moving rather than hurting. We lost just over sixty thousand damage because we were moving.

This is one of the things that is considered a "challenge" feature of raiding. Many, in fact most, raid/instance bosses require us mages to move. Everything from Prince in Karazhan to Mother Whatserface in Black Temple. Tier 5 is the worst, with every single boss requiring movement of some sort.

And that hurts mages a lot. Let's go with the classic mage versus warlock.
When a movement is required, a mage runs. He is spending this time moving, and not on doing damage. His time budget is totally used up by movement. A warlock, comparitively, is still fairly gimped DPS wise, but actually has useful (and not mana-intensive) DoTs/abilities that can be used.
Thus, the warlock can refresh corruption (or whatever), while moving. Thus, only part of their time budget is used by movement, and part is used by damage. Put bluntly, classes that can still deal damage while moving (rogues are an excellent example, as long as they stay in melee range) severely outclass those who cannot. Like mages.

So. Raid bosses force us to move, which makes the encounter harder. This reduces our damage output, which makes the encounter harder.

Genius? Or just arbitrary difficulty generated because thats just how the mechanics work?