I believe it had to do with commonality of our types when we flew the 300 and NG at the same time. The 300 wouldn't do what The NG would in regards to LNAV approaches. Basically the only way to keep it from leveling off at every altitude was to set zeros. The brain trust thought it would be a good idea to use that procedure on every approach for all types thereafter. Thank goodness we weren't allowed to operate the max and 300 at the same time like they wanted.