Redfin is missing 20% of Listings?

I’ve heard a rumor, and I can’t figure out how to verify this using custom searches on Locator, but is it true that 20% of residential properties don’t appear on the MLS feeds to the Redfins and Homevalues of the world (as well as Windermere, John L. Scott, etc.). I was told that a good 1 out of 5 properties don’t have that checkbox selected that gives the MLS permission to ‘advertise on the public web’. First off, I’m surprised that so many agents would bypass the exposure; however, if they are doing it either making individual decisions, or by encouragement from their brokers, the net effect is chilling on the disintermediation plays. Consumers searching for properties using these discounters (or any of the major brokerages web sites), and not having the services of an agent to find the best properties using Locator, are shortchanging themselves.

Can anyone confirm or otherwise clarify this information? If true, it’s a dirty little secret.