What real Druids did is irrelevant, but the mythology around Druids, particularly Irish Celtic mythology (as written down largely by Christian monks/clerics) is profoundly influential on a lot of stuff that leads to what we think of as "wizards". For example, Merlin was Myrddin Wyllt - explicitly a bard - before he became thought of as a wizard.
That's been pointed out several times upthread. How much of the "prophet and a madman" of the original survive in what most players would probably think of as Merlin? Does his Welsh origin matter any more to the Merlin (and Wizard) than the actual historical druid does to what we think of now with the name?
What? What are you even talking about? Gigolo is a job. Jester is a job. Caravan Guard is a job. Musician is a job. Clerk is a job. Scrivener is a job. Night Watch is a job. Soldier is a job. Apothecary is a job. Do I need to go on?
Fighter is a class. A Fighter could be a gigolo, or a caravan guard or even a scrivener. Likewise a Rogue could be any of those things as a job, but would remain a Rogue as a class. The idea that because a Bard doesn't make a good "gigolo" they're not a good Bard class is a total logic failure. It's ridiculous and nonsensical.
Various folks being a stickler on definitions for Bard and Rogue and Wizard got me thinking about those others. The OED seems about as good a place as any to me for the common use of words.
From the OED: Fighter - one who fights (warrior), one employed to fight (a champion, bully).
A ranger, a paladin, a monk, a barbarian, and a rogue all do those things too. Is a fighter who has a special calling any less distinguishable than those? Is that a sign fighter is too broad a name for the fighter class? Or is it a sign that those others should be a sub-class? If one is a scrivener, the usual definition of fighter doesn't seem to apply except to the training they may have received and don't use. If they are not fighting or employed to fight, they are not fighters, are they?
Why is a Rogue even a vaguely good name for a class?
From the OED: Rogue - a dishonest, unprincipled person; a rascal, a scoundrel. A michievous person (disapprovable, but likeable or attractive).
Is an inquisitive who is a detective a rogue by the standard meaning of the word? Is a swashbuckling duelist better described as being a fighter than a rogue?
However I would suggest they're not "music mages" at all and this is in fact your own, somewhat idiosyncratic viewpoint.
From the OED: Bard - An ancient Celtic order of minstrel-poets, whose primary function appears to have been to compose and sing (usually to the harp) verses celebrating the achievements of chiefs and warriors. Still the word for 'poet' in modern Celtic languages; and in Welsh spec. A poet or versifier who has been recognized at the Eisteddfod. In early Lowland Scottish used for: a strolling musician or minstrel (into which the Celtic bard had denegrated, and aainst whom many laws were enacted). Applied to the early versifying minstrels or poets of other nations, before the use of writing, as the Old English gleeman, Scandinavian scald, etc.
The Druid seems to get the magic:
From the OED: Druid - One of an order of men among the ancient Celts of Gaul and Britain, who, according to Caesar were priests or religious ministers and teachers, but who figure in native Irish and Welsh legends as magicians, sorcerers, soothsayers, and the like. Hens in some modern applications: a priest, religious minister, chaplain, or philosophic bard or poet.
One of the paragraphs in the Wikipedia article expresses Tolstoy's theory that Merlin seems more like a druid.
Anyway, googling around has me wanting to put Tim Clarkson's "Scotland's Merlin" on my list of books to read.