The danger when refueling arises if a potential difference develops between the bowser and the aircraft. The action of making fuel flow through a pipe creates a static charge. If this charge exists in the bowser and not the aircraft then nature will try to equalise the potential difference by way of a spark jumping between adjacent surfaces. A miniature lightning bolt if you will. The hose is earthed to the aircraft through metal to metal contact. Belt and braces requirements make an additional earth wire desirable. As aprons are in general poor conductors, the conductivity of the tyres is irrelevant from this viewpoint. Also the earthing is required between the aircraft and bowser and not the earth although this is also achieved during hydrant refuelling.
In the seventies a number of supertankers exploded due to static charges supplying an ignition source during tank cleaning operations.