You're kidding right?
How many times do you see number 19 or 21 running onto the footy field with the starting lineup?
How often is number 4 playing fullback or number 11 is a center.
Just further on this point.
Its easy to tell what position someone is by the number on their back.
Fullback is the guy in the number 1 jersey. Unless he is wearing 4, 6, 19 or 2.
Wingers are numbers 2 (unless he's at fullback) and 5. And 22. And 18. And 16
Each team has 2 centres. They wear jerseys with 3, 4, 13, 19, 21 on the back.
Five eighth is 6, 11 or 18.
Halfback is easy. Its the guy in the number 7 jersey. Except when its 18. Although if its not 7 or 18, its almost certainly 21.
These are all from Rd 13, 2011, and just the backs.
There might be lots of reasons why people want to keep the current numbering system, but it frequently does not represent the position the player is playing on the field. The best way to see what position someone is playing, is to look at their position on the field, not the number on their back.