That's fair enough, and I agree. I'm sure they could have taken some of the weaker members. But a couple of the Shichibukai are at least as strong as the Yonkyou, like Mihawk (for sure) and possibly Kuma and Blackbeard.

But the point is: the WG played a game of divide and conquer with the pirates, in order to get some strong fighters on their side. Even if it is only a temporary alliance.