They are good if you want to work on controlling a heavy weight down and still be able to come up with something manageable..... gives the positive results of a heavy negative while letting you do more than just take it down.
Lots of old timers believed in doing heavy negatives to build the nervous system and get it all used to handling big weights.
Honestly though, I just use bands and chains for this same thing and have never hit a point where I've felt like I needed to walk all the way across the gym to grab the weight releasers.