I proceeded to do the same analysis for shadow bolt. Assuming that I would feel a need to upgrade my bolt once monsters require more than 3 hits to die, I need to upgrade it about once every 4 levels. This is actually a conservative estimate, because it doesn't account for monsters' elemental resistance (which is around 10%, or 5% in my formula) or for the monsters' damage curve which accelerates. 3 hits to kill a monster may not be fast enough for a mage when the monsters can kill you with 1 hit. In any case, once every 4 levels means that 40% of the skill points in the game (my calculation in the post above was a little off) must go towards shadow bolt alone -- otherwise it'll be useless.
I don't know if more skill points could even fix this. I think what's needed is a non-linear curve for all offensive skills that don't base themselves on your current DPS. Instead of gaining a set amount every level, shadow bolt should gain more and more with every level. Of course the amounts have to be balanced, but I think this is the only way the mage classes will become balanced. Also, what has to be accounted for is area effects. Area effects are non-linear by their nature (you hit multiple enemies for the same amount) so they need to accelerate much more slowly.
I'll start working on what I think are proper, accelerating values for the skills.