Point taken on resistances. That being said, I just ran my 3x fire mage party and the only time I ran into anything headache wise with resists were those fire immune undead guys near the witch (so, in practice, in act 1, resists were a non-issue).
But I can see Larin including some hiberheim map in the full game and bricking elemental mages.
Even if common enemies have baseline 10/20/30% resistance, that's still less damage than the physical classes who aren't reliant on abilities with cooldowns to deal their damage.
So if you "build wide" you gimp your already resisted damage even more by not having enough INT. Which contributes to the problem with caster classes doing less damage than their physical counterpart.
I'm of the group that like building their casters as blasters not CC bots. Mages should be able to do both, maybe not both at the same time, but at least being able to build as a blaster and not being less useful than a physical character.
Cause right now, you're better off just building 3 physical dps and have a healer doing only healing and debuff removal/environmental removal.