Well, there is a lot to be said on the subject. In a team based sense the idea is to have highly specialized characters that compliment each other, which is where the "glass cannon" mage idea comes from. The problem tends to mostly become unfair to the mage when you start getting into the idea of hybrid characters like various kinds of holy warriors like Paladins, Shamans, or whatever. In WoW I think this is one of the major problems with the mage and to a lesser extent the rogue, that they are too specialized in a situation where everyone else is some variety of hybrid who can do more than one thing, and one of those things is invariably to DPS as wellas a mage or rogue (or both in the case of a Druid since he has specs for both). In games like D&D (and especially the earlier AD&D) there were lots of ways to make functional warrior-mage types, and depending on the rules being used it wasn't even remotely unbalanced due to all of the subclasses, hybrids, specialty priests, kits, and whatever else.
In the truest sense, from books and such, a mage generally didn't bother to wear armor because he didn't need it. That old guy wasn't too feeble to wear it, it's just that the reason his beard goes down to his ankles is that he spent the last 200 years learning arcane magic and knows how to do things like create force fields. Either that or he's enchanted his much more comfortable robes to provide protection in excess of mundane armor.
In your typical novel all of the main characters (or major supporting characters) who get involved in the cool stuff are uber in some way. The warriors are so buff that they routinely defy the laws of physics when they fight, the rogues are more suave and heavily gimmicked than James Bond, the priest types have an angel on their shoulder (literally), and the wizard fits right in. Chances are being a book the characters all overlap to some extent. That thief is probably also a master swordsman, the priest does more than heal and can call down divine wrath, and the warrior is probably walking around with the +6 sword of "do anything" that covers him in unbeatable plot armor. Assuming they even have those kinds of roles and everyone isn't actually a total polymath to one degree or another. The problem is that what works in a novel doesn't nessicarly translate well into a game, because with all the characterization a lot less action happens over the course of 300 pages or even a 900 page triology than happens in a video game, and what's more being a novel the good guys are set up to win from the very beginning. A Paper and Pencil RPG or Video Game requires both the whole thing to be balanced and present jeopardy, with a chance the good guys are going to lose, and has to involve constant action with tons of fights, traps, and other things being launched like a machine gun. In a novel, it's not unheard of for a hero to eventually just walk right up into the main bad guy's lair and after an appropriatly dramatic seeming confrontation lay the smackdown on him. In a video game or PnP RPG the process is not typically made that simple. The fun comes from dealing with the 60 gajillion obstacles before that, which conversely wouldn't be quite as much fun to read about (differant generes differant requirements).
A lot of games DO find ways of dealing with the issue, especially single player games where a balanced team isn't really all that important. When a player controls the entire party it doesn't much matter of one character is weaker overall than the others, or only useful for very specialized things. In a situation where there is only one player character, it even works for him to be a polymath who does everything to one degree or another (like say in the later "Elder Scrolls" games). You'll notice in a lot of these kinds of games being magic doesn't generally limit your other options, and magic is typically presented as being a lot easier to use, especially for a hero (innate talent or whatever). A player character isn't assumed to spend months or years researching each spell.
At any rate, I'm rambling. I am hoping that with the next generation of fantasy MMORPGs at least the developers will have learned from WoW's mistake, and find some way of making mages more versatile, especially if they plan to have a lot of hybrid characters in play. For example I've been of the opinion for a while that Blizzard should consider getting rid of one of the three trees and create a tank spec for mages to get their defensive spells up to the point of a warrior of equal level's armor, increase their threat generation so they can pretty much walk up into thing's faces and tank. I think it's kind of sad that pretty much every defensive spell they have is like bloody tissue paper, Paladins and Priests should actually be generating less concern when you see a glowing shield around them than a mage, because you know, that's actually one of a mage's quintessential abillities (hurt others, defend self).
