He had one stone explode, but because he far exceeded its capacity.
He's smart enough to know the limits of the stone and OP enough to create far better ones with a much higher limit too.
What he lacks now is a magic bag to always have them ready.

My guess is that those stone will help his assistants. He chooses people with mana, but they will never have levels high enough for his plans, so those stones complement that.
I guess having some stones can help using some power without activating your mana, better stealth attacks