User blog comment:AYET/(99) The worth of the Shichibukai to WG/@comment-3125914-20130419112005

Blackbeard became a Shichibukai because he defeated and captured Ace. Though nobody heard of him at that time, by doing that he proved to be really strong. And he also has the Logia type devil fruit, Yami Yami no Mi.

As for Buggy, I think the WG made him a Shichibukai because of his crew, now that it includes the strong escapees from Impel Down. With such a crew, he might stand a chance against the Yonko, despite him doing nothing. Also, having ties with Roger and Shanks may give others the fake impression that Buggy is strong and worthy to follow. So, it's a matter of how others see him, not of who he really is.