That's not necessarily true. Certain spells in the Netharnan's repertoire would be fairly useless against someone with fire resistance of some sort, but fire resistance does nothing about explosion damage which a couple of the Netharna mage's spells do a lot of. Fireball isn't meant to be a one-hit-kill spell that sets people on fire and burns them to death, it's better suited for suppressing enemies while your archers rain arrows down on them or warriors get close without fear of being taken out before they get into effective range. That doesn't mean that taking a direct hit from a fireball will just bounce off because you have fire resistance, the explosion from it does a fair bit of damage; trying to fight while you're on fire is difficult at best. There's also Rupture, which is literally just a massive explosion that does heavy damage to everything around you. Powering it up with a handful of diamonds via the king's Empower spell makes it a great suicide bombing technique, or can be troublesome for an enemy if fired off from a bit of a distance.
Mage classes aren't supposed to be going toe-to-toe with the warriors or hexblades or other melee classes, they're supposed to stand back and throw spells at them while the rest of their party does the heavy lifting in terms of combat. I know from first hand experience how useful it is to have someone moving around the battlefield firing off tomb spells at the enemy, especially when they're wearing high powered armor.