Position less basketball is a great concept, but the reality is taller players have advantages over shorter players at certain spots on the court and quicker players have advantages in other spots/situations.
So by using the 1-5 indicates physical or basketball skill-sets. Very few players are perfect and can do everything.
Sure Momcilovic can play on the perimeter and shoot 3 on offense all day long. But he doesn't have the quickness to guard small ball 3's. Not a criticism- I wouldn't want Tre King, Kunc, Ossuniyi, Ward or Jones to be the primary defender on a Rice, George, Gdick, OBannon, etc.
IMO being able to switch 1-5 is more coach speak vs reality. Good coaches and players excel when they get mismatches of posts(4/5) on guards (1-3).