I'll bite.
Bugaboo said:
It's not stupid, and please refrain from such insults.
He didn't insult you. He said your idea was stupid (i.e. a bad idea). The judgment is passed on your idea, not on you personally. And if we aren't allowed to pass judgment on people's ideas then there really isn't much point to a discussion board, now is there?
Anyway, this thread was over when it was pointed out that, yes, one's combat role only includes the stuff that one does in combat. I can roleplay my character as an asocial know-it-all (just to grab a popular archetype) and have that be any class - fighter, ranger, wizard, and what the hell, even a bard. But it's not _necessary_ for the purpose of RP to have a party made up of one smartass, one bleeding heart, one or two antiheros, and one spastic.
In contrast, there _is_ a benefit to having a party with at least one each of Defender, Controller, Striker, and Leader. And therefore there is value in assigning those roles names, because it allows for an easy marker by which one groks the combat capabilities of a given class (e.g. someone releases a custom class and says "this is a controller with hints of leader" and you immediately understand what kind of powers it will have).
Combat is the dimension in which _balance_ is most important to design in from the ground up. Having defined roles, and the assumption that all parties will have at least three out of four covered, facilitates balanced design. Roleplay is completely independent of combat role, and one does not depend on the other. You can roleplay with your dice just as easily as you can without.
Example that much of this board will probably easily recognize: the manga Bleach. Ichigo, Orihime, and Chad all have the same overriding motive (which, incidentally, is common to most heroes of shonen manga) - the desire to protect those they love. They have the same main goal, but implemented in totally different ways. In D&D terms, Ichigo is a Striker, Orihime a Leader, and Chad a Defender. Their characterization as different individuals is done as much by their approach to combat as it is by the way they interact with one another out of combat.
tl;dr version: combat role only dictates the role you play in combat. That is the full extent of its purpose.