I have a boom vang and I use it a lot when I race.  It keeps the boom from lifting when on a run thus reducing effective sail area.  It isn't necessary to have one on a M17 but I sometimes use it when recreational sailing (especially when "racing" Catalina 22s *L*), and I like to have it onboard as an emergency back up to the main sheet.

John M Edwards
Miss T
M17 # 372