Encompassing the roles is based on historical accuracy and classic mythologies. In D&D they are using the bard name but not the actual historical reference attached to the name, or the myths surround bards like Myrridin,Taliesin, Vainamoinen, or similarities with classic Greek stories associating magic and music, or the same roles for different cultures like Africa, Persia, China, and Indonesia. It's easy to differentiate bards from wizards because bards were healers and wizards were not. It's easy to separate bards from druids because druids were priest and bards were not, especially looking at the same roles outside of the Celtic roles. It's easy to separate bards from clerics because clerics were not priests and based more on orders of knights devoted to gods and granted magic from those gods while bards were seers who served as advisors and teachers for rulers and who's magic came from the natural gift of inspiration.
It's not clear to me whether or not you're using the class names ("cleric", "bard", "wizard" etc) in their D&D sense or in some other (ordinary Engish?) sense.
As a class, clerics - according to Gygax in his PHB - are inspired by the crusading military orders, whose members were not priests but were lay brothers. But in D&D, clerics always had "priest" as a level title, and are I think are now indelibly associated with the idea of priesthood.
I don't really see how you can say that Vainamoinen is a bard (seems plausible) and that bards are not priests, given that Vainamoinen is a placeholder in the story either for a god or for some sort of ur-priest ancestral figure.
And I don't really grasp the basis on which you're saying wizards are not healers. "Wizard", in D&D terms, doesn't really correlate with any historical or mythological notion - historical witches and wizards can (by reptuation, at least) perform healing as well as other magical feats. There are D&D books that put forward Merlin as an archetype of a wizard, but of course it is just as easy to present Merlin as a druid or a bard. And if we look to literary rather than mythical characters, Gandalf - another archetypical wizard - could just as easily be presented as a bard (he heals, and as a spirit-being is clearly in tune with the music of creation). Elrond is a healer and a loremaster, but except for the D&D demarcations around who can and cannot use healing magic there would be no deep reason to label him a bard rather than a wizard, nor vice versa.
The DDN packet bard that came out is similar to the concepts I mentioned and 3.x, where they are skilled experts and knowledgeable. The main feature of knowledge and skills is there, healing is there, party multiplier abilities are there, spells are there, and they use a mix of spell magic and song magic.
The warrior-skald built using the D&Dnext bard class has d6 HD and is therefore hugely vulnerable in combat. And is a spell caster.
The loremaster is proficient in the rapier and shortsword for no very good story reason that I can see.
The rougish minstrel isn't particularly represented by this bard class at all, other than the rapier and light armour proficiency (which the rogue also gets). In particular, all the social capabilities provided by the College of Wit are expressly called out as magical abilities (except perhaps Seeds of Doubt). As far as non-magical wit and charm, a rogue build is just as viable as a bard build. (The College of Valour actually has more non-magical social abilities, but they're confined within the combat mechanics.) As far as I can tell the bard has no special competence or advantage within the interaction mechanics.
So I personally don't see the D&Dnext bard as covering all the historical and literary archetypes. I see a magician class based around musical/oratory performance and some other support magic, who is also a loremaster and perhaps a fairly weak warrior. It's an oddity of the D&D world, with no real basis in myth and history, that the best loremasters also happen to be rapier duelists (at least until 9th level, at which point the full casters get access to Commune and Contact Other Plane).