This argument is counter to both ABYC TE-4 and the 2017 NFPA marine lightning standards. A GDT/SPD needs to be connected to either the keel or an external lightning plate to be of any use. They are "allowable" under NFPA standards but must be within 8" of the ground point it is connected to, but they are by no means a requirement.
Having surveyed boats hit that have GDT/SPD's, I would suggest saving your money. Why? Because in a lightning strike they can still pass extremely high and damaging voltages, upwards of 300V to 500V, before they actually begin to do their job. This can still wipe out most on-board electronics. Using a GDT on a spar makes little sense anyway as the spar should not be used as a conductor other than for lightning. It should also not be immersed in an electrolyte anyway, so isolating it from galvanic currents, with a GDT/SPD is not really solving anything anyway.
As a lightning damage surveyor/consultant for insurance companies and boat owners I see more hull damage on boats where the mast is not lightning bonded to an external lead keel, iron keel or an immersed lightning ground plate for boats with internal ballast. The only good data we have suggests the same. I also see less lightning strikes on lightning bonded vessels rather than more strikes, what I see in this regard also follows the industry data I have seen as well.
Until the insurers begin requiring lightning damage surveyors to record any on-board lightning protection systems, on the report, the data we get will continue to be poor in regards to lightning systems. I have been working with a couple of the big marine underwriters for a few years trying to get them to begin collecting this data, it's not really all that difficult, but as of yet none of the insurers require it of the surveyor.. I note installed systems on all my reports but this is not a requirement as of yet from any insurer. Some insurers however are actually mandating a lightning bonding system be installed after a strike has occurred..
