>>If one were to start a warmage and never train intel/wisdom/discipline. Outside of the ranks being learned faster in RL time with higher mentals. Is there any penalty for super low mentals?

Well, in short, mentals factor into spell effectiveness, so in a way, yes. Rather than saying a penalty, though, it might be more accurate to say that especially for non-targeted spells, more mentals equate to more effective spell effects (and defenses against said spells).

Furthermore, Discipline, Intel, and Stamina all affect Concentration, which directly affects gweths, familiars, and spell casting.

~player of Gulphphunger

It would be extremely foolish to neglect mentals entirely. They act as skill boosters for some things, for one thing. But beyond that, you'll have much slower advancement over time that higher stats won't offset.