At 10% miscast you'll get about 9 spells per every 8 turns. 5 turns of casting spells gives 10 spells, one of which will fail and disable spellcasting for 3 turns.
It wouldn't be a a horrible trade if it was that predictable. Cast a spell and get a twincast promotion with duration 5 that degrades to a no-cast promotion with duration 3. Unimpressive as a mastery type spell, but a solid level 2 spell. The randomness kills it though.
With another twincast promotion: three turns of spells gives nine spells. First spell turn four disables spellcasting until the end of turn 6. So 9 spells over the course of 6 turns compared to 12 spells without Master of Creation. If it wasn't random it could be a weak adept level spell. (2 turns of twincast, 4 turns of no cast)
Miscast actually disables spellcasting for a random number of turns, I'm using 3 turns as an average.
