[Illuminated One]

Well, I see two problems here, the first is that a normal war won't look like a bunch of troops vs. another bunch of similar troops but some tough sacreds vs some normal troops or some very high damage sacreds against some high def sacreds or something like that.
So any strategy has to be tailored to your enemy and many nations have limited capabilities of adapting their strategy at will. A spell that decreases defense against high prot troops? A mr spell against a high mr enemy. And depending on what your enemy is using the spells that work wonderful in theory don't work actually. So you have limited range and your enemy sticks to the far away corner with his regening sacreds? Guess what, the AI won't cast your spell once you are in range or you are outfatigued by then.
And don't forget that all the tedious work you did to make it work can be negated by different map placement.

The second is that it's often just an illusion that early magic cuts it (*). Yeah, sure, you will win. But only as you are constantly outnumbering your enemy (40 troops + 4 mages is not equal to 40 troops + 1 commander).
A 200 gp mage casting numbness 5 times gives 15 troops -3 to def and att - your enemy can now take these 200 gp and get 20 more troops, equaling to 20 extra attacks at +2 or more attack skill, guess who wins?
This is overly simplistic of course, and there are occasions where you can win from a outnumbered position by clever use of weak magic. It is however entirely possible to win both large and small battles by refusing to use your mages at all before the big standard spells come around.
So before you use it you'll have to figure out if it's actually better than just more soldiers in that situation and often you'll not even have the choice (well, the only way to insta get troops is raising PD where you have to overpay horrible for big battles).

(*) (well, from the numbness example const6 for w bracelets and alt 3 for numbness is not exactly early, so it isn't directly against your statement, but at that time you'll have to expect your enemy to use something as well - getting 2 mages for wrathful skies and a bunch of chaff can be easier that getting 10 mages for numbness)