Worthless isn`t same as useless.
I mean life light can be only used to heal NOT resurrect 5th lvl (using life light to heal any lower lvl is pure waste of mana). Lets say you have 5 5th lvl units (for resurrecting Rune mage is mandatory) , first you need to place them all together and then heal. Then again you need to cast Phantom to resurrect any fallen(more then 1 time probably). So it`s 2 spend turns to gather units around and then presume fight. I rather fight normally, then leave some weak enemy before ressing my troops while picking chests. Any spell have it`s use, but Life light is inconvenient because of small range, and high mana cost to be dmg dealing spell, and it doesnt resurrect

If it worked like shamans ability Magic axes to heal any unit on battle field i would use it first, but now it`s to much trouble to gather units around.And since i`m not big fan of 5th lvl builds i`ll stay with Illusion/resurrecting for now.
Exsorcism is higher dmg magic spell, but again it`s single target for 30 mana, Geyser is 8 target physical for 30 mana. I just now checked 80Int, 2 Gloves of destroyer, Dragon cloak - Exorcism 17,5K dmg, Gayser 8,5-16K dmg. If you have only 1 enemy unit Exorcism is OK, but that is rarely case, so i prefere Gayser for 8 tagrets. As you mentioned physical resistance demons have 10% max, so only problem would be Ghostly units. All other undead/demons take 90-100% from Geyser.
And using Exorcism on summons (x2 dmg to summoned) is just silly, Dispel is better.