Practice Perfect's 4% reduction in failure is equivalent to increased energy cap, which is both far rarer and far more valuable than just increased energy. The resource economy in uma musume is too tight to reliably sit at 0% failure all career long, especially when taking exceptional quality trainings whenever possible. Over a typical run, perfect practice will on average prevent at least one failure, and one failure is worse than losing 2 turns because it removes a (usually high quality) training and replaces it with a wasted turn, a debuff that you have to waste another turn in the infirmary for, and often residual mood debuffs on top of that. Therefore Perfect Practice is equivalent to gaining 2~3 more training successes over the course of the run, or 50~100 stat points.