It reduces your risk of accidentally killing yourself due to a botch, but I'd expect you won't usually be casting to a level that injures yourself, so it won't come into play much. When it does, it may be literally a lifesaver, of course.
Things that give you extra fatigue levels, on the other hand (e.g. shapeshifting into a boar which has 3 extra, or a bear which has two extra in addition to being size +2 and a better combatant), are very useful.
(See also: https://forum.atlas-games.com/t/card-board-games-archive-links/116/1 for an example of a character taking LLSM to ridiculous extremes).