0_o Rei is right, Grass/Dragon has no 4x electric weakness. Quite the opposite it has a resistance that allows it to take only 1/4 damage from electric moves.
Lightning rod tends to be on most Pokémon that either resist or don't take damage from electric moves due to their typing. Honestly I think the only one who benefits from it the most is Seaking.
I am pretty sure it is to resist Thunder Wave which restricts Sceptile's Speed also :)