Also, JP could have 3 x lvl 3 spells before resting when he dings lvl 5 instead of only 2.
For more shenanigance it's good to remember that anyone can store the spells to it and someone else can attune it later and cast them. Misty Step could be nice for Berg to avoid obstacles. Also 3 Guiding Bolts on the stone could be pretty nice for Berg so he could have really good ranged damage. TBH, no matter who has it i would most likely have the stone loaded with Guiding Bolts anyways as it's damn crazy spell for lvl 1 and you can store up to 3 of those.
The stone has pretty much limitless possibilities and it seems to have had rather limited use up till now.