[Twan]

The nature protection spell increase the chance to see your infantry killed by your own fire spells, and has a too long range to position your mages to cast it on archers only (the stupid witch doctors like to cast protection on hoplites, not really increasing their already high prot but giving them -50% FR). Of course once you have marble warriors or army of lead/gold the protection problem is solved but it's probably a little too late in most games (and you are probably using living statues, mechanical men or others summons at this point).