I don't think wizards have quite as much influence on battles as to make them look modern, unless you are in an incredibly high magic world. Your spells effect small areas and run out quickly. If wand and wand users are common place, then it's a different story, but that's not most worlds. A good 'modern magic' soldier would be something like a Ftr 1/ Sor 1 to allow him to use wands, but most NPCs are not supposed to have multiple levels and PC classes.
Having a few mid level wizards would be handy, but not more important than artillery, and artillery has been around as long as warfare has been. (From Greek and Roman scorpians and catapults to modern MGs and howitzers) Those mass pike and musket formations were fighting in a world of cannons and grapeshot. Massing up was still more valuable than dispersing.
If a mass formation is open IE: X-X-X-X where each X and - is a 5' square, then the formation retains all it's utility, but it considerably less vulnerable to fireballs than the XXXX formation most people think of.
Modern battle is really a response to modern communications and aircraft, where any unit that sees a massed enemy unit can call in an airstrike. This requires more magic than anyone but Shark uses for thier fantasy battles. You can fake a gunship or bomber easily enough with a magic carpet and a few mages with wands, but the C3 to direct that carpet is hard to come by in D&D.