My concern with using the Vent Pipe would be wind load breaking whatever seal there is between the vent pipe and the roof. And it's questionable whether or not the vent pipe would be considered a good ground. Hopefully you ran a ground wire from this to your service entrance or other ground rod.
I've seen a similar effect when snow and ice on the roof caused the roof to move down and the vent pipe (PVC in this case) stayed put, causing the seal to be stressed and allowing water to run down the vent pipe. Over the years, that caused enough damage to the block of wood supporting the vent pipe in the basement that it rotted, dropping the vent stack 6" and forming a water block which negated the function of this vent pipe.