[thejeff]

I think you're missing the connection. Fatigue doesn't impact the ability to cast a spell, but the spell also costs a gem/slave for every 100 fatigue. That's how the gem cost is determined.
Thus "A B3 spell that costs 400 fatigue can't actually be cast by a B3 caster", not because he'll have too much fatigue, but because 400 fatigue means it costs 4 slaves and a B3 mage can only use 3.