I'll say it flat out, it really depends. If you want to use the refugee system, fine, i'm glad its making a come back actually. Its handy and helps avoid unwanted conflicts or helps a player who feels wronged get a city.
However, lets be honest, if a top end player is local to us, good/has proven themselves, wants to join, and is under attack. Then you're probably going to laugh someone out the door if they try that refugee stuff. It only applies for lower end players. Its a simple trade in most scenarios, give up the player, or give up the alliance. Most rational leaders know which to pick.