Use-It-or-Lose-It
Use-it-or-lose-it is the principle that skills and mental models decay when they are not retrieved and used. Munger includes the idea because possessing a concept in theory is not the same as being able to apply it under pressure.
Why Munger Includes It
Munger's whole project depends on active recall. A mental-models-latticework is valuable only if its models come to mind at the right moment. Knowledge that sits inert in books or memory without being exercised becomes decorative. It feels owned but does not participate in action.
This is one of the cleanest links between Munger and the wiki's learning-science cluster. spaced-repetition, retrieval practice, checklists, and decision journals all serve the same end: they keep important patterns alive enough to be available when stakes are real.
The Practical Lesson
Do not confuse accumulation with accessibility. Reading widely matters, but so does revisiting, applying, and stress-testing what you have read. If a model never gets used, it fades. If it gets used often, it stops being trivia and becomes judgment.