Not to mention the fact that the world is a very large place, and 100 million people in "Golarion" doesn't mean that they're all available to you within an easy walk from Absalom.
I think it's funny how, when blasting is usually considered a sub optimal thing to do, everyone seems to think that a high level spell caster is going to waste his time blasting at level 4 and lower soldiers.
A high level caster is far more likely to be throwing down Walls, fogs, and pits to control opposing troop movement, summoning large groups of creatures, or throwing around Dispels to neutralize opposing casters. Hell, even low level illusions would generate a bigger effect on the field than metamagic'd fireballs.
There's a difference between frying five Kobolds, and immolating thirty Hobgoblin spearmen, with Fireball.
That said, yes, most battlefield mages would spend a lot of their time countering their opposites on the battlefield, but if you can bring one mage, you can probably bring two or four. And if one side has more mages, that extra can cause some havoc even though he's likely to be the lowest-level out of the bunch.
Another consideration is the idea of battlefield roles. Your highest-level mage is likely to get the most use out of Fly, Greater Invisibility, and the Dispel Magics before she gets to cast a single offensive spell. A Mid-level mage is likely to be a specialist who is loaded with three or four spells geared towards one or two predetermined tasks (You toss attack spells at their flank, you summon monsters to shore up weak points in the line, you defend the general/king, you take a team and hit their casters, etc...).