When my son was 16 he was trying to choose a travel team. The head coach of one team suggested to him he throw from three or four different arm slots.
That was the last time I let my son talk to that coach.
There may be some that can do it, but who? Most who can are side-armers. How many slots do the overwhelming majority of MLB pitchers throw from? One.
The reason is deception and a repeatable delivery. If you can throw 3-4 pitches and they all look like a fastball you're money in the bank.