But...why would a coach take somebody mid season ...
Two good reasons, one bad ....
Good - You're trying to build the best possible team to qualify for nationals and do well there, this player will take you to another level AND you've made your philosophy very clear to parents and players from Day 1. (Not a fan of those kinds of teams, but if there is honesty in communication, then the families and players can make that choice.)
Good - You don't have enough players.
Bad - You have enough players but you care less about the ones you have and more about your own need to win.