Not quite sure of your question. The runeword works no matter who uses it as Frostburn said. There's good reason for light sorcs to carry a low strength version themselves. The additional -45-55% enemy lightgning resistance helps against monsters with high light resistance.
If you were wondering about non-lightning based sorcs. It will up the damage for fire based sorcs nicely. But there isn't any -enemy fire resitance so let your merc carry it. It won't break the immunity of of 90% of the fire immunes out there though. For cold sorcs it does almost nothing. You already keep monsters at near the -100% resistance limit so no help really. It won't break 99% of the cold immunes either (Ok, maybe I'm exagerating, more like 98%).