When my mage grows up a bit (Or if you are a solo mage) I often send them off scouting in really tough areas where I know my fighters will get toasted.
When I cast lightning, some times it fries my mage. Mage Robe of Electrical Resistance and Boots of Grounding prevent this. Mage Robe of Fire Resistance is handy for the reasons I already mentioned. Mage Robe of Cold Resistance is utterly useless in BG1.
As for Lightning, why do I use it? I really used to hate the spell. There are some fire resistant critters in BG. Or critters that shrug off fireballs. A well aimed lightning bolt with bounce around a good bit and criss cross the same enemy a few times. A well aimed and careful shot could be devistating, to both enemies and your self, always make sure your mage works alone when casting.
The Skeleton Warriors in the Thieves Maze are just about the worst fight in the game. I find a thief or a mage valuable here. Go invisible, try to avoid the traps, and cast fireball or two or three, or, a nice lightning spell will fry them quickly.