I think the reasoning behind that was that, as a Grass/Dragon type, Mega Sceptile only takes 1/4 normal damage from Electric attacks, and gains a small boost in return.
Loool which is useless. Lightning Rod immunizes the users from getting hit by Electric moves, and no it doesn't act as the Justified Ability. XD
There is actually a logical reason why Mega Sceptile has this ability, because its tail is based on a conifer tree and they are known to draw lightning to them. The ability does have its competitive use as well, especially in double and triple battles where it can easily protect any Water or Flying-type teammates that usually get threatened by Electric-types.