The use of reverse is for most of the manufactors SOP, meaning after landing you must engage the REV at least at IDLE, otherwise you will have an event and you gonna be called to the office. AIPs and legislators can say and write what they want, because whatever they may write it can never be against safety. Other point to note is, looking into past events, we learn that some fatal crashes happened from overrun after REV being held at IDLE and not fully used. Why this happen? After so many years of using IDLE rev all the time, your muscle memory is used to IDLE rev and thats it. In the event you have an unexpected failure and need to act on the fly, you will be hostage of startle effect during which you will be using your muscle memory before you can think clearly. Unfortunatly, when things go south, everything happens very quickly.
So, the industry evolved, and now most of it recommends to, uppon landing, move the power to full reverse and in case you dont need it, bring it back to idle.