[Lewdcifer]

I'm bad at balance so this might probably sound like a bad suggestion, but it's worth a shot.

We all know that Spellthiefs normally have a cooldown on their stolen spells of three turns.
Currently, there's not much advantage to main classing Spellthiefs, and the cooldown imposed on stolen spells sorta make them not so much worth it.

Which brings us to my suggestion:
What if main classing Spellthief reduced the cooldown of stolen spells to two turns (or one)?
This could make them more worth using, and would give Spellthiefs more reasons to be used as a main class.

[Autumn]

It might be okay, as long as it doesn't make spells with cooldowns at already 3 rounds 2 rounds instead, it should be fine.

[Snake]

I agree with the OP, Spellthief is in a state where it can get more useful at the trade of big stuff like Fleur, High Speed Divine Word or Weathered Body.

And, yeah, though that will probably not be intentional anyway, and most likely a bug to be reported if it happens.