This was a nice topic to read. In all my role playing, I have truly hated spells that actually increase a stat's score. Instead, I have always altered the spell so that it gave a bonus to using the stat instead of actually increasing the stat. I have always found that the stat bonus addition worked better than the stat increase.
Reasoning: Power-user says, "I can make your agility easier to use, but I cannot actually improve your agility without also performing a transmogrification to physically change your body."
And if you physically change a person to improve their agility (example), then you will also affect the other physical stats of quickness, strength, and constitution. Thus, if you physically change a person to improve their agility, you might end up actually lowering their strength.
Make sense?
rmfr