Wizards that can cast improved inviso and fly can also cast protection from arrows, so he wouldn't be all that worried about archers.
Still, that's a trio of spells, so the amount of artillery spells available will be less. And providing artillery is what the wizard on the battlefield does -- just like regular artillery. Howitzers can easily be destroyed by a few well-placed AP machine gun rounds, and, as a result, they are not placed on the front line, but back where they can be protected. In a like manner, the general who sends his wizards flying over the enemy risks an important asset.
Fact is, a wizard on the battlefield is a potent weapon -- few can compete in the ability to kill scrubs. If the enemy army is all scrubs, you don't even need the improved invisibility... If the enemy army has spellcasters or anything that flies, that's a dead wizard.
I was running a wizard when the orc army was coming to attack the kingdom. I found a herd of cattle, hired folks to herd them into the path of the advancing army, and then cast 5 different versions of contagion on the cattle. The army came along, ate the fresh meat, and died soon thereafter. Who needed fireball?
OfficeRonin