I believe a lot of the extra PP spells, and the spell mastery SCSM stuff, relates to the fact that many spells are not inherently scalable, and yet many are.
All those X/Lv effects cause the spells to inherently scale up with caster level, at no PP cost. . .
And the bolt spells scale up with directed spell skill, so a high level bolt is likely to be more dangerous than a low level one, due to the higher OB the caster will have, but no effect with balls.