When EPL switched to a squad number system from the 1-11 system, merchandise sales went through the roof. As mentioned, it allows you to put player names on the back of shirts as well. So it's a good idea.
What the Super League system was was a basketball style pick your own number system. This is crap. We should adopt the squad number system, which is used in the English Super League. That is, you have a squad of 25, at the start of the year you allocate numbers out to the 25 players, 1-25. Your first choice fullback gets number 1, and so on. People outside the top 25 get the next available number (26, 27 etc).
This squad number system also used to be used on Kangaroo Tours. So it definitely has a history in RL.