I don't think you understand the basic concept of class quests.
It doesn't change that it is available. Just because you don't give a shit about the class, and therefore don't play it, doesn't change that it is available. If you don't care about the class enough to have it at level 100, which is what is necessary to start doing the class quests, then you obviously don't care enough about the class to begin with.
Now there are two things that really bother me about your attitude.
1) If we go that far, we basically can't even have artifact weapons in the way we do. It already wouldn't be ok to have a quest where a DK goes into Icecrown Citadel to retrieve the remains of Frostmourne to reforge the Blade and claim it for himself. That would already not be ok according to you, because that is a class quest that only a DK can do, you can't do that on your mage, so we can't have that.
Do you really think it's fair that the community loses out on cool quests like that, just because you don't want to play a DK?
2) Complaining that you don't get to do DK quests on your mage, is like if I complained that I didn't get to do Horde quests on my Alliance characters, and I don't have a Horde character, therefore Alliance and Horde should always have the same quests. But some of the coolest content is faction-specific. The Forsaken quests in Howling Fjord are some of the most memorable faction-specific content. But nope, we can't have that, because it isn't "available" to everyone. It's unfair to force players to have a Horde character in order to see the Horde content.
Sorry, but your opinion is awful.