There's not much of this horse left to beat, but here goes.
While the FAA's regs & AC43 should always be consulted, that doesn't say they are always perfect.
Bonding of the antenna to the skin requires a tight, gas tight joint (and I'm not talking about the sealant around the edges). Try this experiment: smear some type of 'witness' paste (conductive paste, or even grease or vasoline could be used) on the base of an antenna, and then mount it to a 'skin'. A scrap of aluminum sheet would do fine for the experiment. Tighten the screws to spec. Now remove the screws, pull the antenna loose, and observe where the antenna base actually contacted the aluminum sheet. You'll see that it only contacted the skin reliably at the points directly under/around the screws.
A star washer between skin and nut on each screw, and another under the head of each screw, will provide more effective bonding than any level of bare skin.
And that big area of bare skin under the base is an invitation for corrosion; moisture can enter the area through the screw holes in the skin from inside the structure.
Charlie