I just did my 5-year last calendar year and was not required to retake the MMPI.
You are right, procedure wise it technically isn't required unless you fall in one of the categories below. Not every utility is going to be the same of course but some require it for whatever reason it may be. Like I've said before they can always go above and beyond any required procedures if they feel fit to do so. For whatever reason a lot of people tend to like to retake it after they haven't in a while or just want to get out of work for a little while because the MMPI isn't timed it just has to be finished in one day. Some utilities are talking about switching to the PAI which is a different test with about 350 questions.. but I'm not sure how far that has gotten.
(1) Individuals who have extensive knowledge of defensive strategies and design and/or implementation of the plant’s defense strategies, including—
(i) Site security supervisors;
(ii) Site security managers;
(iii) Security training instructors; and
(iv) Corporate security managers;
(2) Individuals in a position to grant an applicant unescorted access or unescorted access authorization, including site access authorization managers;
(3) Individuals assigned a duty to search for contraband or other items that could be used to commit radiological sabotage ( i.e., weapons, explosives, incendiary devices);
(4) Individuals who have access, extensive knowledge, or administrative control over plant digital computer and communication systems and networks as identified in § 73.54, including—
(i) Plant network systems administrators;
(ii) IT personnel who are responsible for securing plant networks; or
(5) Individuals qualified for and assigned duties as: armed security officers, armed responders, alarm station operators, response team leaders, and armorers as defined in the licensee’s or applicant’s Physical Security Plan; and reactor operators, senior reactor operators and non-licensed operators. Non-licensed operators include those individuals responsible for the operation of plant systems and components, as directed by a reactor operator or senior reactor operator. A non-licensed operator also includes individuals who monitor plant instrumentation and equipment and principally perform their duties outside of the control room.