If you can afford the better units such as a Fox Pro or Minaska Bandit, they are a great tool. Totally necassary, NO. But they do give a great boost to success especially for beginners that may not be able to produce the right sounds and tempo of hand calls. With that said, I have a FX3 Fox Pro, but rarely use it. If I were hunting thicker cover, that would probably change because they do come in handy, especially when hunting alone, to get the sound away from you and to help pull the coyote into the area that you're wanting to get the shot at. When hunting with a partner, you can do as well with hand calls, simply have a shooter sit up to watch the downwind side and have the caller sit where he can cover the other direction.
Those E-callers aren't cheap, BUT they are effective. I just don't really need one and I can do as well with hand calls.