If you've got the money to shell out for a good unit, like a FoxPro, they are very good tools to have. But if you don't, they aren't necessary, and you can spend $40-50 bucks on a couple of mouth calls, practice with them a little, and they will work just as well.

I've got a FoxPro FX3, and it is a good unit. It has a remote that allows me to switch sounds, control the volume, and start/stop the call. I set it out and start it for 15 seconds or so, then stop it for a few minutes, then start it up again. Predators' ears are very quick, and it doesn't take them long to locate the sound and come running.

I don't hunt much from blinds, especially when using mouth calls. The blind muffles/deflects the sound too much for my liking. I carry a camo folding chair to where I am going to call and then set up so as to minimize my outline (I also usually wear 3-d camo). I try to set up so that I can either limit the coyotes ability to approach me from downwind, or I can see downwind further than he can probably get a good sniff of me.