When is granular erythritol as good as powdered?
The crystals are only really useful when you want a crystalline form, for instance sprinkling on top of sugar cookies where you want that crunchy sugar texture. I sprinkle raw sugar crystals on top of some cocktails as well (when there's a froth on top to support them) as a garnish, I imagine you could use them that way as well for a lower calorie version.
For any application where you want it mixed in the crystals are not better than powdered, as you point out they are hard to mix in.