Basically, it comes down to the fact that any tool can be used incorrectly and detrimentally in the wrong hands. These are not innately "evil" collars. They are tools and can help reinforce commands that are used in working situations. They can also be used to create a lasting aversion to life-threatening situations like chasing deer while hunting or investigating a deadly snake. (Those aversions are created by high level shock at precise timing; you can imagine then that using too high a level at the wrong moment can make a dog timid about something you hadn't meant to be associated.) Hence, they are collars that should be used with care and consideration, as well as knowledge on when/how to apply them.
E-collars have their uses as described in previous posts, such as sporting dogs. We use them on our hunting dogs, although we use a vibrate option more then the shock, both of which are highly adjustable, as well as "nick" option which is very brief. None of our dogs are cowardly or fearful when we put the collars on them, nor in general due to the use of them.
Most pet owners do not learn how to train with or use e-collars properly, and true they can create or worsen issues if not used correctly, with the right type of introduction, level, and timing. Like Tess described above, sometimes they are used too harshly, and yes, even professionals can misuse them.
I think it mostly depends on the experience and preferred techniques of the trainers who are using them, as the hunting dog kennels I've been to and watched work in the field had no dogs that showed any timidness or nervous behavior. All were excited to go work and were enthusiastic in the field, including one of my own, trained at one of these kennels.
If you're having issues at home, there's probably another way than getting an e-collar to handle it effectively, and I would suggest finding one
Too many buy an e-collar and think they can just shock their dog when they see it doing something wrong, out of the blue and inconsistantly. Or they skip steps in teaching the command (which you generally show/teach without the e-collar).