I have seen dogs spook and RUN AWAY when shocked. So while I do understand how some people would want them for 'extra' help when off leash... I have seen it back fire if the dog is not listening cause its spooked (vs chasing a squirrel)
And the fact that some dogs can ignore the shock just as much as a recall when they are prey drive. If I don't think I can call my dogs back.. I don't let them off leash.
With Alley, the ecollar work was taught on a long line. I taught Nyx an ecollar recall, but really haven't taught her ecollar for crittering. I'm not sure whether or not I will. So far, I have ended up not needing it. But I don't fully trust her off leash. So the ecollar is on her in case she gets away from me and doesn't come. If she's chasing something in prey drive, I don't know that I would use the collar, since she hasn't been taught it in that context. But when she's gotten loose, it's been to go do laps around the yard, not to chase. She did once get loose when there were geese in the yard. I grabbed a tug and she saw that. She decided mom and tug were more interesting than running geese. I was very happy about that, but I'm not yet ready to trust her enough to take the ecollar off. I really would like to get her to herding training and show her how to properly use her drive instead.