A calcium sequestering agent is needed for carbonate developers and also for developers that contain a large amount of sodium sulfite. In the first case calcium carbonate may precipitate and in the second case calcium sulfite.
While borates cannot be used with catechol and pyrogallol, pyrophosphates are ok.
Don't worry about any of the extra ingredients in Dektol and other commercial versions of developers. Kodak is not hiding a magical ingredient from the user. Some "extras" are used to achieve a stable powder mix, others to help the stabilility of the mixed developer, etc. For example, Microdol contained an antisludging agent.