Longeing has been an invaluable tool for my mom and I while retraining our spooky/hot warmblood mare. The point wasn't to "tire her out" but to refocus her mind and get her paying attention... sort of priming her for when we get on her and tell her what to do from there. It has helped her start to rebalance herself in a much nicer way of going, and after so many years, she's good at it! She responds almost perfectly to our voice commands, enough so that it is sort of fun to ask her to do certain things and see her go at it.
Some people tend to use longeing as a means of wearing the horse out, but we used it to engage our horse's mind. When done properly, the person longeing should be paying constant attention to the movement and gait of the horse, refining even the pace at which they travel in each gait. It's never a good idea to make a horse go around more than two or three laps in any gait at a time. Transitions transitions transitions are key to keeping them engaged and paying attention to you, waiting for what comes next.