If the best group to accomplish something consisted of 6 rogues, would that group be balanced? Of course not. Everquest is built with balanced groups as a design goal.
Even if 6 average equipped classes in a balanced group can kill Gorenaire (which of course, they can not), 6 wizards being able to do the same thing is still wrong. There's no way to justify it, just as there would be no way of justifying 6 rogues as the best possible group in any other situation, or the best raid being 50 warriors. As soon as a group requires 6 of the same class, that class is unbalanced - regardless of the circumstances.
Pretend for a moment you're a designer, developing content. You would like to add some content that isn't like Luclin content. In other words, you would like to add some content that's actually exciting and fun, something like fights were in the original game with Vox and Nagafen, or even fights in Kunark. Fights where you don't need a book or a television to survive it.
However, you find quickly that you can't make this fight. Why? Because manaburn makes it trivial.
No matter how much damage that mob does, no matter how resistant it is, no matter what is done - manaburn will take it down in 6 seconds. Sure the room may have a lot of other mobs in it, so you have to have other people there besides the wizards, and that's great - but the fight itself will be trivial.
Even if the mob has twice as many hp as the 8 or so wizards expected to be at a raid can do...those wizards are still overpowered for that encounter. Those 8 wizards do as much damage as the rest of the raid combined. If the raid has 16 wizards, no others will be needed at all.
There are three solutions to these two problems:
Oh, and team manaburn? Ours has earned about half a million plat, each, solely through the use of this one ability. Quite a deal for 5 AA points.
I'm sure rogues can use their Poison Mastery skill to make money just as fast, and warriors can save that kind of money with their Bandage Wound skill. Since these UBER money making skills cost 18 points instead of 5, I'm sure they make money 3.6x as fast...right?