There is absolutely no reason whatsoever for another crew to leave an alliance, as the loot is not divided between crews, just given to everyone. The function of leaving an alliance is merely to troll and grief other players.
Perhaps if once an alliance was broken, the people who turn in the loot got a bonus then i wouldn't have complained, as the game would provide an incentive to de-ally and not a troll button. As it stands now, there is a large incentive to ally with other ships and not break the alliance, except to people who solely play to cause people to become upset.
TL:DR Leave alliance is for trolls and to fix accidental alliances.
Side rant: Don't use the it's a pirate game excuse or "you shouldn't trust strangers!" Why shouldn't I? Because its a pirate game? No. There is no reason why anyone should break an alliance with another ship if they helped do a task. This is because the game doesn't award this behaviour in any way that would warrant a person to do so. I understand accidentally allying as that happened with my ship. But if two or more ships helped get loot together then they shouldn't break the alliance. Rare themselves have stated in developer updates that they were happy about the way the community worked together during the Hungering Deep and bilge rat adventures. The reason I'm so aggresive in this post is because the game was on the way towards a more positive community, but now it might sink back to the betas and launch days of sink on sight instead of the past couple days of asking if help is needed.
