At which point, even the most magic hating Nord will get the idea, and attack the mages. Common sense isn't a magic power, though it may seem so considering it's rarity.
It's not common sense, it's basic knowledge of conjuration doesn't mean that everyone that know nothing about magic will know. If I don't know anything it's possible that I'll think that the conjured creature would stays even after I killed the caster.
But alright, I'll give you another one, Let's say the dominion use necromancy, if you've mages they can cast turn undead, etc and thus lower casualties than attacking them.
Another one the dominion uses invisibility to retreat, a mage can cast detect life to prevent them from escaping, what can non-mages do?
And yet another one, the dominion use levitate spell so they'll be harder to reach if you''ve mages they can cast dispel.
And another one, a frenzy spell has been cast, if there's a mage you can cast calm spell to counter this, with no mage? What can you do?
This is but speculation, but I'm pretty sure any decent army would adapt. If they realized their tactics against magic practicioners weren't working they'd soon recrute some for their own ranks. If TES VI or some future DLC were to deal with the invasion of an independent SKyrim, I'm sure we'd be seeing a lot of magic practicing Nords and allied forces. But this is speculation, all of this.
Nords are known to be stubborn.