However, I'm not convinced, at all, how this is useful, still. If the Staff of the Magi needs an arcane magic user to wield it, what is gained by saying "needs to be a mage" over "needs to be wielded by an arcane magic user"?
I'm no longer in the game of trying to convince anybody of anything in this thread, but I will at least point out that in the latest packet, the term "arcane magic user" isn't explicitly defined. Unless I missed something, the class description for Mage (which will likely be renamed back to Wizard) begins with "As a student of arcane magic," and it has a class feature called Arcane Tradition. Is this enough for the Wizard to be an "arcane magic user"? What if they change the flavor text and rename "Arcane Tradition" to "Wizardry School"? I suppose they could introduce the 4E notion of power source, but how different is "Power Source: Arcane" from "Class Group: Mage"?
In my opinion, the answer to the last question is that 4E drew a formal distinction between Power Source and Role. Monday's article reveals the current thinking to be that "mages" are physically vulnerable, while "warriors" are the toughest. Even in 4E we quickly learned that arcane defenders had a splash of controller, and divine defenders had a splash of leader. In other words, some power sources were oriented toward particular roles.
Personally, I'd rather they bring back Role and Power Source as independent labels (and be more disciplined regarding the temptation to fill in the matrix for its own sake), but we know a substantial part of the player base strongly associates e.g. healing with divine magic. In this sense, saying that priests' magic "can heal or protect their allies" is acknowledging a property of the standard D&D world, for better or for worse.
Anyway, as a design element, class groups present tradeoffs. Sure, maybe a cleric of a war god should qualify for a "dwarven defender" prestige class on the basis of having proficiency with all martial weapons. But if the prestige class required "warrior" instead of martial weapon proficiencies, then a monk can qualify for "dwarven defender". (Remember that we don't have attack bonus anymore.)
Anyway, I don't want to give anyone the impression that I'm convinced that "class groups" will necessarily be a positive contribution to the game. We really haven't seen enough to know. It's an idea they've floated. I can see some merits and some risks. As some people say: As always, play what you like.
