When is granular erythritol as good as powdered? Is there any recipe for which granular erythritol (i.e., crystals) is at least as useful as powdered erythritol (i.e., very finely ground)?
Cooking with granular erythritol usually requires dissolving it. In my experience, erythritol doesn't dissolve in fat at all, and it barely dissolves in water: a saturated solution tastes only mildly sweet.
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.