I've always used them. When I worked as a part time installer, at a two way radio shop, I had to replace damaged antenna's at various times. Many times a previous installer didn't install the rubber gasket in conjunction with the antenna. I would often find where moisture had made it's way under the antenna and made it's way under the base of the antenna and wicked it's way up the threads of the large nut. I've had to replace whole coax assemblies/NMO mounts, after customer complaints of noise, reduced reception, etc., due to corrosion developing where the antenna made contact with the mount. In 100% of the installs, on my personal vehicles, I've never had an issue with moisture wicking it's way up the threads of the nut when I used one of these gaskets.
If an NMO mount is installed properly there should be no moisture leaking past the rubber "O" ring under the large nut. I use a silicone(?) grease, made by Motorola, on the rubber "O" ring of the nut and the rubber gasket anytime I do an install.