Mage is the only class worth playing in any of these games in my opinion, they always seem to have more impact on the whole story than any of the two remainder classes.
1.) In DAO, A mage warden is likely the only Warden that is a mage, as Duncan states the wardens only ever have one mage. So Duncan taking one likely means they were in need of a Warden Mage. Only a Mage Warden can enter the fade to save Conner. A Warden Mage has a bigger connection to the happenings in Broken Circle. A Warden mage is also connected to the poisoning of Arl Eamon via helping Jowan escape the tower.
2.) In DA2, while Bethany is a much beloved character of mine. Mage Hawke would seem obviously more sympathetic to the mage situation in Kirkwall if not for 2 of his companions being mages but him/herself being a mage and the fact that their lives were shaped as outsiders on the run simply because they and their father/sister had magic, and thus be that much more driven to spark the Mage Rebellion. Though I suppose if you went with the Templars either of the other 2 classes would work as well since magic is the thing that seems to cause Hawke the most pain in his/her life.
3.) In DAI, being given a magical mark to do magic with seems like something a mage would better be able to understand and utilize, i mean it was intended for a mage to have in the first place albeit Solas, who in a case of bad judgement gave it to Corypheus, in turn lost it to the Inquisitor.
in all three mage is seen as a negative thing amongst the populous and by championing the right cause and saving the people you in my opinion help to show mages in a better light. Sure the Rogue or Fighter can become king or queen of a kingdom, or a paragon and thats all fine and good if the story ended there, but a mages story is more open ended, they actively have their freedoms revoked, still suffer under the chantry, there is just more resistance to mages making their way in the world and that leads to better stories than, "Fighter X appeared and saved the day and the people cheered." a mage would be seen like, "Some mage came along and fixed the problem that other mages caused, so we'll give them one more chance but they better watch it." the noose for magic is still their, magic as a whole is still seen as too dangerous and thus more tension for future stories remain.
then again thats just how i see it.