Reading through the trap the soul spell on the wiki I find the fact that only one resultant gems being useful a bit silly.
One possible solution would be to make the creatures trapped within the gems usable as summoned monsters, similar to the one shot summon clickies given out in the festivals, active for 10 minutes then vanish to their eternal rest. Obviously this would lock out other summons while active.
Either that or more crafting options that use the gems would be great, but the above options would be even better.