I do not think that  assistants should be trained in scaling patients' teeth.  Without proper dental hygiene training the assistant may provide substandard care. If they would really like to scale teeth they should enter a certified dental hygiene program and become a hygienist!!  There are many things a hygienist does as part of a  "routine "scaling appointment --  intra-oral extra-oral exam, checking periodontal health, medical history assessment, and much more-- scaling should not just be an entity on it's own.  It is part of a comprehensive treatment plan.  As the level and quality of education goes down, the quality of care to patients will surely suffer.  We should always strive for the highest quality of care for each patient we treat and that includes  scalings done  by a dentist or dental hygienist.