I have been involved in that a lot - but 10%? oh yeah, in Brisbane the team is 5 senior, 5 junior and 11(?) open. thats only 5 slots, unless there is a lot of others trying out.
but like you also said, the most of the players are already in the field anyway, they just do this to get them grouped together. its not as if anyones missing out on a time, unless thats their pet time