Why would it be a liability?
If I make an error and get called to the carpet, how I would get dealt with holding an ATP is of a different standard than as a CP which is a different standard from PP. Example being my gear up: Had I had a PP, it would have been over with at the scene, competency questions addressed and resolved with a discussion. As a CP it required a 20 minute, 3 lap 709 ride. As an ATP I would have certificate action on my record as well. Ergo the liability is a certificate action; what that is actually worth to me is questionable, but I see no reason for spending money to assume it when I don't need to. I don't gain ratings for the ego thing, I gain them when I have some use. My boss at the time paid my MES because he had a twin Otter on amphibs for the program, but it never came into use.