So okay, what was the purpose of Starswirl the Bearded's unfinished spell?
If the unfinished one switched all the cutie marks, what does the complete one do now? It's pretty much a spell to use if you don't want to switch cutie marks all up, right? whu-huh?

CONSIDER this...

He must have tried his own spell... It activated the Elements of Harmony, and switched the c-marks of those who yielded them a thousand years ago, right?
And he NEVER FIGURED IT OUT.
Hmmmmmm...