User blog comment:AYET/(95) The Last Shichibukai/@comment-3125914-20130325134632

Interesting facts. You pointed out really well that Marco has all the qualifications for a Shichibukai. However, I don't think the Whitebeard Pirates are so weak so that they can't defend themselves and need protection. Even without Whitebeard himself, the crew has a great strength. So, basically, Marco has no reason to join the Shichibukai.