Like most everything in fly angling, it depends...
... on the situation the new student angler is in.
I wouldn't for a moment consider teaching someone to fly fish on the South Fork of the Snake starting with a Tenkara outfit.
But on Birch Creek, Tenkara would be a great way to get some one started with several advantages over conventional fly angling gear.
Just two of so many examples that would illustrate the point - that it depends. Of course, in either the case, the teacher should be reasonably competent in using both approaches.
John