I can't comment on kids with genuine disorders that make spinners a useful tool.
For the rest of the kids though they're certainly a fad toy. I tend to believe that if these kids have that much pent up energy and feel some need to tinker with little toys it's likely because they're not getting that release elsewhere. Depending on age and their interests I'd happily buy some LEGO, a model car/airplane, some crafts, teach some culinary skills, start a garden, teach them to change the oil on the car, do some woodworking, etc. Any number of useful, skill building things that get them off the computer, away from the TV, and off the video games. Kids are a lot smarter and more capable than many give them credit for and I really can't blame them for being bored and restless.