I do not doubt the existence of evil--whether it be war, pestilence, slavery, torture, pain, hunger, etc., et al. It exists.
I do not envision some devil afflicting mankind with evil, like MacBeth's witches, conjuring evil from a magic cauldron.
However, judging from their rhetoric, Islamic extremists do. They envision Satan in some manichean struggle with Allah. Therefore, for reasons of communication, it is sometime best to use their own symbolism. Maybe they'll understand that language.
Furthermore, sometimes an ounce of symbolism saves a ton of explanation.