I worry about the impact for sellers of this "pocket listings" trend

I would surely want to discuss this carefully with my clients before keeping a home off the MLS. I've seen too many demonstrations of the MLS listing working to Flagstaff home owner's advantage. The market price cannot be really determined in a private sale.

In other words, the ONLY legitimate reason I can see for a real estate agent to NOT list the property on MLS before allowing his or her seller client to make a contract to sell is if there are extreme privacy concerns of the seller. Then, I would want to make sure the seller understood the advantages of an MLS listing in order to (temporarily) preserve that privacy.

See: MLSs at ‘cliff’s edge’ over "pocket listings" from Inman News.