I've been doing the BCP thing now for 8 years and for my goals and love of pond management I couldn't be happier. The statement of "..it all depends." fits here real well, imo. For some pond owners in your area they may be working fine for their goals and your fish seller is basing his recommendations from those experiences.
Or, like you stated, he's just trying to sell an excess inventory of fish he can't get rid of
