I could comment to this, but only from my personal perspective. I speak only for me.
I want my choice to be meaningful. My role in the game world is important to me...I want to feel heroic, powerful, and really LIVE my role choice. It is my profession, my ultimate skill set...it defines me.
Your role (meleeDPS/ rangedDPS/ Tank/ Heal) is ultimately defined by your specialisation. In game mechanics the defining feature of Advanced Class is to isolate base healing and tanking abilities, and to reduce role selection to a selection of 3.
Allowing a change of that role would trivialize the choice for me. It would not longer as special or meaningful to me as it once was.
But you can already change that role. In fact, if you are a subscriber, have purchased field respecialisation, and play an Advanced Class limited to roles that use the same gear (for example Sorcerer, rangedDPS <-> Heal), you can change your role at no cost, at nearly any time (not in WZs if I recall), and without having to swap out gear (if you are a perfectionist you might, but accuracy, power, surge, crit all get the job done for these two)
I will not be happy if this is implemented, but naturally I will not oppose something that the majority wants....if this is what the majority wants I will live with it. But if someone asks my opinion I give it.
[edit: I should clarify, I'm not trying to shoot you down on a personal level, I really want to understand the argument from both sides.
For me the meaning in character comes from the root class, the appearance taken and the decisions made (LS/DS) as they level. The abilities are merely tools to get the job done.]