Some PC's have said, "Well she forces the follow through but look how good she is". I always counter with, "But just think how great she could be if she was pitching with a natural arm movement instead of a forced one".
In my experience, there are at least two reasons many PCs teach "hello elbow" to their students. Either a) they don't really understand arm whip and how to generate velocity and impart spin to the ball using proper pitching mechanics; and/or b) they want their student to see immediate success (that might be achieved short term for newbie pitchers) otherwise they fear losing their student and their student's $$ if they struggle for several months.
My DD's first (now former) instructor was one of those "push the ball down the arm circle" and "finish with the pitching hand coming up to the pitching shoulder after release" kind of instructor that are all too common in fastpitch. These instructors get paid a lot of money over the years from unsuspecting parents; you would at least think the instructors would actually spend a few minutes watching some slo-mo pitching video of Jennie Finch, Amanda Scarborough, Cat Osterman etc. to see if what they are teaching is actually how good pitchers pitch. They would quickly realize that the correct ball position and arm action during the pitching motion of quality pitchers is the exact opposite of what these instructors are teaching. It borders on criminal (or at least gross negligence) in my opinion.