Most spellcasters that have sparks, frostbite or flames, will not use use them until they are low on magicka at higher levels.
J'zargo on the other hand will use sparks against enemies in sparks range and won't use his other spells until enemies are out of spark range.
Which is 90% of the time since sparks has a pretty decent range.
J'zargo also won't use weapons in melee, he prefers to use his fists.
I managed to fix this with an npc editor and now he uses lightning cloak, thunderbolt all the time.
He still uses his fists in melee, but I gave him the fists of steel perk so he actually does mediocre damage in melee.