It's also noticeable that you don't really need to invoke Merlin's half-fiend nature, or Gandalf's angelic status, to make a D&D-style spellcaster that can do the sort of things they do. In fact Merlin can be done without several whole schools of magic, if you make him a Wizard, and still match up to most of the things he does in the tales (flying Stonehenge across from Ireland might be a little hard).
You can look at this with most of the classes, in a way. They're wide enough in scope to cover a wide range of heroic archetypes. Wizard covers a huge range of spellcaster types, who are often in practice specialised in a few sorts of magic, and gives access to them all. Rogue covers con-men, pickpockets, spies, scouts, burglars, detectives, and a lot of other skilled and cunning types. Fighter covers people who use weapons really well - and disregards the warriors who have something extra about them, whether it's extraordinary abilities or leadership talents.