Disadvantage on a
cantrip would be ok to do at-will. And roughly equivalent to advantage on an attack. Maybe a single attack from a character who gets most of his damage potential by multi-attacking. But a single big-damage attack or an attack with a rider?
But disadvantage on dominate monster, would not be good to have at-will.
Currently, if a creature has disadvantage on his saves for whatever reason, it applies to any save, doesn't it? It is, afterall, an affect on the disadvantaged creature.
It seems to me like casting Dominate rather than Hold rather than Sacred Flame (or whatever) is a resource-management decisions, the difference in power is all based on that decision. Choosing a victim less able to resist is just part of making a good decision. It doesn't matter if disadvantage is caused by a daily, at-will, or improvised action, once you've expended the resource to take advantage of that disadvantage, the resource is gone.