User blog comment:LTMJR/BLIND SIDE TO WHO JOINS LUFFY CREW/@comment-24769684-20170125162425/@comment-24769684-20170125224938

Well allying is different. Luffy allied with Crocodile, one of the series' worser villains, because he knew Crocodile's help could benefit. The Curly Hats reluctantly allied with Caesar himself when they needed his help in battle (though it could be said that they pressed him into service). Guys like that certainly won't join the crew, but they're not out of the question as allies.