As I said above, I am always in favor of a release, but there are some valid points made above about the rest of the girls left behind especially if there is now no longer enough to field a team.
As for reasons for unhappiness, that can open up a whole can of worms. Of course, some coaches overcommit up front on things like playing time or schedule difficulty, and other coaches will get blamed no matter what ... they play favorites, they bat their own DD too high in the order, they don't know what they're doing, they're being overly critical of the girls, they don't practice enough, they practice too little, DD isn't playing her best position, etc. Too often, it seems like there is another coach out there who is promising something better, whether they just desperately need players or they're trying to improve their team ... and then you find out later that the player/family left wasn't happy there either, and the following year they are looking for yet another organization.
I would also wager to say that the majority of the time when someone asks for a release, that it's actually the parent(s) and not the girl who thinks the grass will be greener elsewhere. As others have mentioned, as long as the fees have essentially already been spent and/or can't be recovered elsewhere, there should be no refunds. But yeah, I don't want a family that is unhappy, and I am going to sign a release every time so that kid can go play somewhere else if they choose to.