Obviously, real life battles did not involve magic, but stop to think what would have happened if they did.  Archery replaced javelins because it was more effective.  Guns replaced arrows because they worked better.  A fireball seems a whole lot more effective to me than an arrow.  Why then would you have this older technology around when it was obsolete?  The answer to this is that mages are rare and anyone can learn to use a bow.  Unless you have some way of keeping every single player from making some kind of mage variant, however, this argument fails.  How about the ability to summon creatures behind the enemy flank?  Or the ability to feed an entire army with magically summoned food?  If magic is to be used in any sort of viable yet interesting world, it must be limited.

In personal combat, this is doubly so.  Asheron’s Call annoys a lot of people, but it is the logical progression of magic as a technology.  If chainmail is lighter and stronger than brigandine, everyone who needs armor will try to buy that.  If all armor is not as effective as a buffed robe, everybody will be a mage.  If there is any one combination that is superior, everyone will use that combination.  Add to this the idea of a world in which the most profitable job is killing things and there is an endless supply of stuff to kill (stuff that doesn’t get any smarter), then this is what people will do.