I have time to answer just now, but GorkAndMork completely nailed the things with much more depth. I would just add a little bit to it answering to your post.
What you are talking about is/was not a bad idea in optimal circumstances. However in reality it caused a lot of dependencies, limitations and more factors to fail as a raid. You had to bring such classes to provide the raidwide buffs, 1 single player had to be present to increase the dps of a whole raid (even if it was a 10 man, and also 10 man raids were much harder to do than 25 mans, because of the more limited slots).
What if a player providing the raidwide buff dies in the middle of a progress fight, granting the reduced raid dps and reaching enrage. Or he/she just simply doesn't appear on raid, because of irl things.
Nowadays we have no need for such buffs and self-sacrifice for the greater good and it makes the raid more powerful and flexible this way, because you can still beat the enrage timer when 1 or 2 dps die. Each has the same amount of resposibility (other than some specific tasks, but in that case there still can be backup members) which makes the whole group stronger than before.
And also as GorkAndMork said, individual competition in a raid is real, and I say it's a good thing, because it motivates players to perform better and better each time a fight starts.
Hurray! I've missed my good old battle shout. <3
Anything that can help prevent class stacking is good in my book.
World needs more Goblin Warriors https://i.imgur.com/WKs8aJA.jpg