@Jack:
Yeah, the schichibukai are helping the WG doing…. nothing.
Think about the resources it takes to fight these pirates if they're not on the World Government's side. With them as allies, they're not sending whatever Vice Admirals/Admirals on a dangerous assignment to get rid of these pirates that, as shown in the story, are willing to work with the World Government under certain circumstances. Does someone like Doflamingo help the World Government in the same way the Marines do? No. But the forces that would have to be used on pirates that can be negotiated with, are instead spent on fighting pirates in the New World and the 4 Emperors. And obviously in situations like the war, they are used in their most basic sense, of fighting side by side with the Marines.
You gotta remember, the balance of power is a balance in a completely chaotic world. It's mainly separated islands, with a good chunk being apart of a World Government. Piracy is a constant threat even the 4 Blue Seas, where the Marines have presumably the most control compared to the rest of the world. The Grandline is a big mess with the Marines having great control in some islands, no presence in others, and every shade in between, which gets even crazier with the New World.
It makes perfect sense in story that the evil corrupt would decide to persuade the pirates that are strong enough to give them extra power in a pinch to be friends, under the condition that they pretty much pardon them and all their actions and turn their heads. It's a win win for them, they don't sacrifice strong forces that can instead fight the pirates that can't be persuaded to ally-up and have grown into a legitimate threat to the WG's power.
So the "balance" isn't necessarily about them getting 7 pirates with the intention of actively fighting on their behalf(which you kind of are insuating by saying that they're doing nothing to help the WG), but rather it's them getting 7 problems out of the way to deal with 4 much bigger problems.