I use the mirror every single time I take off into the hover and also before touching down.
Too many times have pilots got airborne with a sling attached to the hook but forgot about it......
Many times when landing in the bush, the skids may become snagged by a root or something else and a quick check in the mirror will assure you if the skids are free or not. The mirror becomes quite useful when landing on logpads or checking passengers when they unload the tailboom.