So forgive my ignorance on this one, but it would seem that power attacks offer no extra damage, only the chance to shield break without taking a knockback?

Power attacks have nothing to do with finishers, and don't affect sneak attacks. Once you hit enough Stamina to flail wildly with your weapon of choice, I think there's no other reason besides more storage...

Bumping this because I'm genuinely interested...

I think there are a number of bugs like this that I hope (and expect) will be addressed soon. Dual wielding spells, for example, drains magicka faster but appears to do no additional damage.