"good" spells aren't nescessarily a good act
Remember that just because a spell has a "good" descriptor doesn't mean that casting it is automatically a "good" act.
Example:
Binky the sorcerer: I summon a monster to eat the cleric of Yondalla.
DM: That's an evil act you know...
Binky: But I'm summoning a celestial dire lion to do it...it's a "good" spell, so that balances it out to neutral, right?
If you summon a celestial dire bear ("good"), but have it eat some peasants, then it is obviously not a "good" act. While summoning a fiendish dire ape to rescue a peasant from a burning house isn't that bad. An evil sorcerer can not "improve" his/her moral standing just by summoning a bunch of archons.
The way I see it:
Cast a good spell to do good=good
Cast a good spell to do evil=evil
Cast an evil spell to do evil=evil
Casting an evil spell to do good=neutral
I also don't see it as some sort of magical taint that happens (as in: you cast a few evil summoning spells, you get 15 'evil points' which changes your alignment), but a personality assessment. If I were DMing and a character were to constantly cast spells with the evil descriptor, I would say "why is your character calling up infernal energies so much? What attracts him to evil like that? I don't think he's Lawful Good, like you have written on your character sheet, instead he's more neutral...")