If my observations are correct, intelligence does not affect spell damage, only weapon damage.
So why would a "pure mage", one who does not use weapons need int?
The answer is, only to get mana regeneration. I made a post to crunch some numbers on how much int you would need, and the answer is, at least for the beginning and middle of the game, surprisingly few points. You should take 37 points of spirit before you take the second point of int.
So where should all your points go?
str, dex and int are all bad choices, as they do not give you either damage or regeneration.
Vitality or spirit on the other hand, are prime choices.

Here is a (theoretical) level 15 character:
vit=31
str=1
spi=31
int=1
dex=1
HP/mana=337

Perhaps you do not need that much HP, but that is besides the point. The point is, intelligence is a lategame investment, which is contrary to the advice given ingame.

comments?