I've damaged a few pi's and such using step down regulators that aren't fixed output. As LiPo battery voltage drops, so does the output of the regulator. So, adding a pair of nice, big linear regulators to the module which can be enabled / disabled from the micro. Linear regulators are significantly cheaper than fixed output buck converters, and so long as power draw isn't ridiculous, and heat is managed properly, they're perfectly useable.