Negative descriptions of raiding in WoW abound: Mario, dancing, Guitar Hero. Note that all three of those are fun in their own context, but some people feel they just don't fit in WoW.
But why must we dance? Let me put it this way: We do what we must because we can.
Let's start with a few assumptions. First off, we're playing WoW. That means aggro, tanks, healers, DPS. So while we can imagine games with dynamic awesome fights where no one has to heal, that's not going to do anything in the WoW raiding game. Second, bosses cannot be too repetitive, both within a fight and between fights. Boss 5 shouldn't be a reskinned repeat of boss 1.
We can begin with the basic boss: it has an aggro table and it hits the person at the top (minus the 10/30% target switch requirement). It has health and that health is reduced by attacking it. This is the tank and spank boss. It is the template, the core of all other bosses. With enough tinkering they won't be obvious, but the core mechanics of WoW (aggro and the holy trinity) make this the starting point. There, we have the first boss done. And maybe we can have another tank and spank later in, the 20th boss of the expansion. But not too many, because let's face it, these bosses are pretty boring.
Let's add some mechanics to change this up a bit. First off, hard and soft enrages (different bosses). For the hard, let's say in five minutes the boss kills everywhere, so now we have a DPS race. For the soft, let's go with a stacking DoT on the raid which will eventually overwhelm the healers, but which can be healed through for a while, so it's a DPS race, but without quite as clear of an end point. The tuning on this can vary: stack healers to buy time as the cost of DPS or stack DPS and try to outrun the inevitable collapse of healer. Those are the extremes, but somewhere in the middle is the illusion of balance.
Is this boss fun yet? Of course not. Nothing changed. Everyone still does exactly what they did before.
We could add some adds. These could be tough enough to need tanks or just something to burn quickly. So now we at least have a target switch and maybe some tanks run around a bit, but melee DPS probably stay on the boss and ranged just turn in place.
Let's get everyone moving. We can add fires. We can add "boss is angry and will kill everyone within X yards" mechanics. Now we're moving.
Carry on stacking on mechanics in various mixes until you have enough to make every boss you need. For this raid. Then there's the next raid and the next and then an expansion or three.
We don't want raiders to get bored, so we cannot simply reskin the previous boss and his mechanics. Certain mechanics are incompatible, such as a raid-wide root and range-limited AoE. Or are they? This is where addons and dances come into play. Imagine a boss that does a random raid-wide root that does some damage. Inconvenient and will mean some extra strain on healers if people get rooted in fires. Now add in an AoE that goes 30 yards out from the boss. If both are random these can mean that with bad luck a raid can lose all the tanks, melee DPS, and probably a lot of ranged as well. But if the AoE is predictable, then we make an addon to tell us to all run out before it and before the root, giving us enough time that even with bad luck on a root we'll still get out before the AoE. This is where boss mods come from: they allow otherwise incompatible mechanics to be used, thereby expanding the potential bosses.
But dancing, why must we dance? Because there are only so many random mechanics we can respond to. At some point random mechanics are incompatible and so we must have predictable mechanics. Predictable means preemptive action: scripted dancing.
We could just have fewer boss types. Maybe we'll end up reusing Adds with Fire and Soft Enrage. For most people that might not be so bad. It means they have a pretty good idea of what they're doing and the previous fight actually taught them something worth remembering. But if you raid for new and exciting challenges, that reskinned Adds with Fire and Soft Enrage is a waste of a boss. So Blizzard throws another mechanic on top and if that mechanic does not fit, they make it predictable so it does fit. And so we dance.
There are also balance reasons for the dances, but that's for another day, specifically whenever I get around to finishing that post.