Definitely not harmful if the specification was implemented correctly. Literally the only thing that the zwave repair utility does is tell each individual device to update its neighbor tables.
As far as if it’s needed, if all of your zwave devices are zwave plus devices it’s not strictly needed, but it can take a long time before the explorer frames get everything on the network updated. It’s just much more efficient to run a Z wave repair as part of your new device installation process.
Some of the confusion may come from the reason for running the zwave repair at that time. It’s not necessarily to update the neighbor table on the brand new device. But if you did a “bench pairing,” that is, pairing the device close to the hub and then moving it to its desired location, the zwave repair utility will make sure that it knows who its true neighbors are.
Second, the zwave repair will cause the rest of the devices on the network to become aware of the new device, which doesn’t typically happen when you just add a new device to the network. It can take days for explorer frames to find it and get everybody updated if you didn’t run the utility. And if you were adding the new device in order to strengthen the mesh and have it act as a repeater for older network members, then you probably want to see that improvement as soon as possible.
So running the zwave repair after you add a new device and have put it in its desired location is just a way to speed up The process of updating the network tables. You don’t have to do it, but it can definitely help.
Every once in a while smartthings support will tell someone who is having problems not to run a zwave repair “or it might make things worse.”
As far as I can tell, that’s because of the cloud component of your smartthings account. If they are out of sync or, heaven forbid, if your cloud account is corrupted, then running the zwave repair might cause them to become even more out of sync. That’s a flaw in the platform implementation – – it should not be a problem, regardless of the size of your network.
Most smartthings competitors recommend running a zwave repair frequently. It should always be a “can’t hurt, might help” utility. Vera automatically runs one every night. Homeseer recommends running one once a week just as part of regular maintenance.
However, once you toss the cloud piece in, who knows. If you are working with support on a specific problem, I would follow their recommendations during the troubleshooting process for that problem.
But other than that, even with Explorer frames it’s a good idea to run one after bench pairings or after adding a new repeating device.