Kind of like hunting turkeys, I decoy or the lack a decoy can work for or against you sometimes. I've seen coyote shy away from decoys and I've seen 'em come so hard to decoys that makes a guy wish he had a shotgun. A decoy is just another thing to carry and really just is NOT a necassary piece of gear to kill coyotes.
If you set up correctly, you'll get the coyotes out where you can see them, without a decoy.