I think most nurseries would frown on customers pulling plants out of pots. If the plant has been recently repotted, the roots will be disturbed. I agree it would be nice to know the condition of the roots, but feel your advice is not fair to the nursery.
I work in a nursery and just love smart buyers who ask if they can look at the roots of shrubs, trees, and plants. Those who want to look are generally experienced enough to check the soil moisture before tipping the plant out. It isn't a great idea to tip a really wet plant bottoms up. Makes a mess and can injure the roots. However, once a plant has drained a bit, sure, why shouldn't customers examine the root zones of plants they intend to spend their hard earned money on? If a buyer is unsure of how to perform this procedure, he or she should certainly ask for assistance. I check the roots of every plant I buy and, if a plant in the nursery where I work looks puny, we definitely check the root zone to see what needs doing to correct the problem.
There is an exception to not buying root bound plants, however, and that is with perennials.
In the nursery, we are sometimes forced to shear top growth, root prune, and repot herbaceous perennials that become root bound. While they are recovering, we set them in a shady area. In fact, one very famous grower in particular uses an extremely rich mix in their too small pots. This initially creates a lovely lush impression and so attracts early spring customers. However, the plants can very rapidly outgrow their pots, sometimes as early as mid June!
So, every year I use my employee discount to bring a few lovely but root bound plants home, prune their tops and bottoms, put them in a slightly larger pot in shade to recover for a couple of weeks, and then plant them out in my own yard. I've taught many of my customers to do the same with wonderful results.
50 years ago I worked at a nursery. The owner was just as interested in what was below ground as above. He regularly inspected plants' roots both when buying and selling. He stood behind his plants. His grandson is still in business with an excellent reputation.