Spamming every skill that's not on cooldown will do buttfuck DPS for many weapons, btw. Because besides Skill 1, usually only one or two other skills will do good damage off cooldown. The other skills either do less damage while doing an additional effect, or are situationally defensive, or are just situationally unique.
If you are a Warrior using a longbow with a bunch of people taking one a single boss, you are wasting your damage if you use skill 3 or skill 5. Because what good would an AOE or an immobilize (that does less damage than Skill 1) do on a single boss while you have a zerg?
Thieves don't have cooldowns. What are you gonna spam?
Guardian with Hammer, have fun spamming Skill 5 that does no damage, Skill 4 does less dmg than Skill 1 and launches your enemy (and probably won't budge the boss), keep spamming that Skill 3 immobilizing that boss that's not moving in the first place, again doing less damage than Skill 1.
Also, mega supa boss fights truly have various siege weapons, destructible walls, destructible doors, open area supply camps, suppy caravans that move supplies from camps to keeps. Yeah.
This reminds me of a Boss on the Norn area, he turned on a shield that reflected damage from time to time, it was just too hilarious seeing all those ranged guys go down like flies one after the other.
There's a Mesmer utility skill that puts a reflecting shield around your enemy. Lulz.