Take a razor and make slices around the root ball. Roots always try to go to water. So the dry medium forces them into the moister native soil. I agree with blueberry I would never touch the roots when they have leaves. I have seen others use your idea while dormant with good success. In fact I have on accident done it a few times. There wasn’t enough root to pull the dirt out.
One other thing that can go wrong besides root girdling with large potted trees …. is that a potting soil heavy on bark and peat or pine fines and peat will decay in time, and in a windy location that lack of dirt (a cavity) around the trunk area can cause the tree to become loose in the ground and blow over…unless the roots have really spread out into heavy clay or dense soil and hold the tree in place well.
It hasn’t kept me from buying trees in nursery pots, but it can and does happen from time to time.
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