It is necessary...read the manufacturer's instructions - they are quite explicit. The thinset fills in any minor imperfections on the floor and provides a fully supported decoupling layer. Not doing it voids any warranty from the manufacturer and risks failure. Also make sure he uses the specified alkaline resistant fiberglass reinforcing tape on the seams and leaves a 1/4" at the edges of the room.
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Jim DeBruycker
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.
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