In my opinion scaling should not be allowed by dental assistants.  I have been a dental hygienist 36 years and often scale and root plane with many successful outcomes over the years for my perio patients but the dentist I work for was also previously a dental hygienist and wouldn't you know sometimes I have been known to leave some calculus!  Dental assistants are so very valuable in other areas to increase production.  I do not believe expanding their duties to scaling as well would be in the best interest of the patients.  KN