So, this is my source for Blizzard's philosophy behind group roles.
https://web.archive.org/web/20041026...l&t=470726&p=1
On the second page, you can see Caydiem reiterating an important point, "The team is aware that some classes are not quite at the point where they match these descriptions. As I said in the closing paragraph, when that is the case, they're working on bringing them up to that point." From this, you can see it was Blizzard's intent to have the classes actually follow that philosophy. This isn't mere class fantasy.
Finally, Tier 1, Tier 2, and Tier 2.5 all had stats to benefit all specs of said class. The tier sets were changed a little before Blackwing Lair release. And, i believe in the same patch, Tier 2 was moved out of Molten Core bosses. At the time, Molten Core bosses could drop both Tier 1 and Tier 2.
This is why i say that Molten Core, and it being poorly designed, unintentionally caused all hybrid classes to be forced into the healer role. Now that you see my evidence, i would suggest you read that post.
https://web.archive.org/web/20050207...html?setid=211