I just wanted to say that I believe that it went off topic really because of the way I worded things.
I did not mean that you should always bring the best class for the role. I only meant that in order for 'Bring the player, not the class' to hold true a certain amount of homogenous is needed.
My team is a tight-knit one and would never consider bringing a different class because of a certain fight, that wasn't what I was saying at all. I do however strongly believe that all classes should be able to perform all roles equally and that no healer should have terrible AoE healing or terrible single target healing.
Cataclysm made some great changes in that regard, there is no healer left behind and I support Blizzard continuing to do so in MoP.
Aplologies for making this thread go off topic and will keep it to just changes in future.