I don't know where to begin.
1 magic scales with level, intelligence and skill. Getting items that boost those is way more important.
2. Mordus is undead, of course he has 200% resist to poison. Every undead does. They also take piercing damage from healing.
3. Mages do continuous damage through surfaces. If you're upset fireball does only 100 damage, but ignore people took 70 from walking out of the fire and applied burning which did more. You're not doing proper analysis of mage contributions.
4. There's so many ways to utilise environmental interactions, like using smoke to block los, oil, frozen terrain, etc that mages can be hard to quantify for value.
5. A caster only party will likely want a summoner to do physical damage with
6. Ideal party is 2 casters 2 physical look at armour values and priorities targets. It may actually be best to have 4 character's who do a bit of both to some degree. E.g. support mage who summons. Battlemage with physical and a backup school. A huntsman with elemental arrows and some magic and an offensive mage who can poly or whatever. Just so you can consistently distribute damage to targets.
I'm constantly annoyed at how the only pure physical character on my team can't do damage to characters with stripped magic armour likewise, when my dedicated offensive mage can only explode someone who's got an exposed physical armour.
People who don't run balanced teams must have huge issues with tons of gear they just can't use. I imagine they'd be constantly complaining about how they need to spend ages on itemisation and gearing people up to get better gear.
