@Drake_Cloud:
Also, you have to weight the Shichibukai against what they would be doing without the title. Its not like removing the title removes Doflamingo's influence on the WG and in the underworld, or stops Kuma from being a revolutionary (in fact the WG got a pretty good deal out of that one).
The Gorosei play a dangerous game by trusting them, but whether or not it ultimately works to the WG's benefit in the end is impossible to tell without a better idea of the New World's political situation.
It has to be beneficial to them, why not make it the seven bounty hunters, or the seven assassins, or the seven mid ranks marines who are pretty strong but don't play nice to climb up ranks and are more effective this way?
Why pirates? To say "look, I have control over these wild beasts!", to take a power out of the equation and to kind of their side (even if they are more likely than not to bite them back), real world reasons don't aply because there were tons of privateers and rival nations, and unreliable communications.
Also, is this particular generation troublesome? I don't feel like searching for their apointment dates, but there are things that we don't know, like when were they formed, as response to Roger's influence, or they are an older institution, what was the initial line-up, what allowed them to be seven, how good did the predecesors do their jobs?