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.
I have found that granular erythritol gives a pleasant crunch to pie crusts. GdD also pointed out that it's good for garnish.
The main problem with erythritol is that it doesn't dissolve well in cold water. But it turns out that you can dissolve a lot of it in boiling water: almost a 1:1 volume ratio (e.g., one cup of erythritol dissolves in one cup of boiling water). And much of the erythritol stays dissolved even after it cools.
Granular erythritol is also great for melting, so you can pour it over something (e.g., candied fruit). Erythritol has a relatively low melting point of 121°C, just a little hotter than boiling water.