That's actually a pretty good point... in the magic-mod I'm making (and may or may not ever release) I'm making battlefield-effect spells generally more expensive, and not reducing their research level, as I think that several are already too strong. Army of gold / lead / marble warriors / fog warriors / haste / relief are particularly suspect... especially in combination with each other! I had been thinking more about mass-evocations in my response.
As far as cheap units with powerful buffs go, I guess they're a pretty potent strategy to which I had not given much thought. Such strategies only work on the offensive (meaning, when you correctly allocate your gems and script your mages), and only when your offensive goes as planned, as I learned in SOC1 when
somebody's Ghost Riders absorbed all 45+ of my mage-gems before the real battle
But they will still probably work.