Can someone enlighten me (no pun intended) as to whether these things actually do anything useful. My way of thinking is that they belong in the same category as snake oil, antenna lacquer or coax polish, but I'm open to answers.
They can provide transverse mode protection; limiting the voltage across the transmission line. But, when dealing with a lightning discharge I'd personally want a darn good common point
ground system in place to drain energy to earth before the cables enter my shack!
One problem with gas arrestors is when the SWR rises due to higher impedance antenna loads... in some instances a high SWR can cause those devices to "fire," which in turn can cause serious damage to a power amplifier. Once the arrestor fires, the amp is running into a dead short. Good bye to the bandswitch, etc.