Leg muscles are HUGH oxygen consumers. If we are striving for efficiency, consume less oxygen. Even an efficient kick is not an efficient way to create forward motion, so adding a 6 BK to an otherwise 2BK swim will instantly decrease the efficiency even if it increases the speed. It’s like adding a red sock to your white load…it’s gonna come out pink no matter what, even if it’s just one sock. Adding any extra leg movement might make you faster but it’s going to decrease efficiency.
if you are interested in RACING, you want to go the fastest you can for that distance which means that on race day, efficiency is a secondary measure…speed is first.
But in the long run you want to determine the most efficient way to travel the speed and distance you are targeting. For a 100m racer this is a completely different ballgame than an Olympic distance triathlete vs a 5k Open water recreational swimmer vs. a 5k OW AG competitor.
Finally, as someone above mentioned, kicking often compensates for balance problems. We try to teach the swimmer to solve these balance problems with lower energy cost actions like getting the recovery arm forward and not stuck behind, opening up the axilla/shifting the scapula forward, optimizing head position etc.
Not everyone can have optimal balance a) with a kick or b) with a new 2bk. However we see swimmers of all body types from long & lean to short & stocky including TRUE SINKERS (those wholes whole body density is greater than water…very rare) all able to establish balance with a 2BK.
If they then want to add a 4 or 6 beat kick to the mix and see what happens, that kick is suddenly going to become an efficient kick instead of a balance compensating kick.