I'm not sure what the numbers are. I think what it amounts to is how committed are they to whiskey. If whiskey is only an occasional thing, then it might make more sense to use commercial grist.
On the other hand, Koval here in Chicago makes nothing but grain spirits, using a bunch of different grains, and they do not mill.
Everybody has different ideas about what they're doing and where their value-added is, different visions.