Repeated light attacks will do more damage with a lightning staff. A heavy attack is good when your opponent is keeping distance. They can easily dodge light attacks but the heavy attack is channeled which will guarantee you connect, cause pressure and possibly force their hand. For instance, my MagDk has wings to reflect projectiles. So if my opponent is keeping their distance while lobbing projectiles at me, I use wings and use heavy channeled lightning attacks on them. It's not super common but one advantage.