I'm curious - The state of the world as it is.. Why would Cancellers or Mindscours be valued at all?
Cancellers can weaken Darklings. A powerful Canceller is essentially a force multiplier for an entire battle, and some of the more epic-scale Darklings are pretty close to unkillable,. Also, there are really obvious social problems that stem from having the most oppressed people in your city pushed into (as far as the cities know) magical black boxes that kill or maim 90% of them and make the other 10% superhuman.
Mindscours aren't
valued so much as they're
powerful. Not only can they read minds and replace memories (meaning they're almost inherently at the top of any social hierarchy), they can steal skills and knowledge, meaning if you can find enough experts, you can potentially be expert-tier in
everything. Even if you're limited to regular human skills, having a few hundred years of subjective skill honing is pretty huge. Presumably in terms of raw administrative and diplomatic skill they're pretty good, as well.
They're not well-liked, and they rarely leave Rosolin. However, Rosolin is also the most organised of the Three Cities - it has a regimented military of Longshots and Breathtakers who can slaughter Darklings en masse, and who possess tremendous co-ordination and mobility. It also happens to be the only city with mind mages, which seems awfully coincidental.
Conversely, Everett barely has a government (powerful elemental mages defend the walls, plant mages feed the populace, and the food can always be tuned to only support a certain number of people), and Madalgia is more like an amalgamation of two to three groups that hate one another, with one group using the poor as a meat shield and defending the outer walls properly only if it's really necessary.
Lastly, since people don't get to choose what power they get, there's no reason to ever turn away a mage.