Overwatch! That is a big reason for Tesla being useful on immortals. Do the math again, assuming you can only roll 6s to hit.
10 immortals, 10 shots, 1.67 6s, 5 hits
10 immortals, 20 shots 3.33 6s, 3.33 Hits
If you are using the Immortals in a defensive position, then tesla just deals a few extra overwatch wounds, which can prevent an asault. Add a lord with gauntlet of Fire to that unit, and you have a durable objective holder. Give him a Res Orb, and it will take alot of fire to bring down you objective holder.
Immortals arent meant to die. You are supposed to have warriors for hte sucicde missions, they go after Land riaders, Immortals hold objectives, and therefore win games.
While Gauss is nice, Necrons have so much reliable AT, that having the ability to put down a couple of Gaunts with overwatch can allow the immortals to win a combat.