@MiyamotoMusashi:
I agree with CrystalShip, at least partly. BinSquarr, you said that all of the Shichibukai have powers to destory entire armies but except for Hancock i don´t really see that. All Shichibukai (except Moria, i really don´t know that he was doing among them) are extremely hard to beat in an open fight, like a one-on-one. You are right that 99% of the pirates in the One Piece world wouldn´t dare to even touch a Shichibukai let alone dream of beating one but the way i see it, none of the Shichibukai have abilities (again except Hancock) which deals with many opponents at the same time. Mihawk is the most powerful swordsman in the world but his sword skills don´t effect many opponents. DoFlamingo can control almost everyone (even someone as powerful as VAs or Jozu) but again, he can´t control an entire army. The same practically applies to all Shichibukai. The only one-man-armies in OP were/are Roger(Garp mentioned that Roger destroyed entire armies just for speaking bad about his nakama), Rayleigh, WB, the Yonkou, Ace, Enel and people with Haoushoku in general.
Besides, strength isn´t the only criteria that is considered for a Shichibukai. The reputation is as least as much important as strength. Moria had an encounter with one of the Yonkou and at least survived, the same goes for Crocodile, Mihawk besides being known as the best swordsman could even get a tie with an Emperor, Kuma was known as the Tyrant, Hancock was rumored to turn everyone to stone on sea.
But with pxss weak bounties like 80/81 mill., their individual reputations on the whole couldn't actually count for too much. What's more, the Supernovae, as you've said, have already overtaken most Shichibukai with their bounties alone. I know that bounties don't count for much, but in terms of reputation, it surely must act as a weighty element. Right?
I'm saying that the Shichibukai've been enlisted because they can specifically destroy armies and avoid getting fxcked up at the same time. Crocodile may've been defeated by Luffy, but that's after he kicked his ass twice. Otherwise, the man could control the whole island, which was composed almost entirely of sand, with his powers. We've seen the devastating effects of 'Sables' on the world class battlefield at Marineford, he makes NW pirates and top marines disappear. He can also grow back his head.
Doflamingo may not be able to control more than one thing at a time, but the fact that he could use a character the size of Oars Jr. as his own puppet negates this necessity. He could just roll up, control the toughest guy on the battlefield and use them as his fist. Then decapitate them after he's done with them.
Moriah himself controlled an army. That was a factor in his power. He can also avoid all hits because of his shadow switching ability.
Jinbei can take out battleships and shxt. We didn't see much in Marineford but that's because there was no water readily available. If there was all the strongest characters (i.e DF users) would be fxcked.
BB also can overcome the powers of DFs. You shouldn't underestimate this when many armies can be reduced to their knees by only a few DF users.
Kuma's untouchable due to his nityu nityu no mi. No one can near him or touch him. What's any force gonna' do?
Mihawk, may I repeat, is the strongest swordsman in the world. He could fxck up any number of cxnts in a single slash. 'Cept Buggy.
Law can control the space around him, so like Kuma, no one can actually touch him (w/o sea stone/haki… blah, blah, blah). His power, like all of the above, is an immediately overwhelming and overpowering force. Hardly anyone can touch these guys, and even if they have a chance to they're probably already getting bum-raped by the one of the Shichibukai.
Fxck these NW pirates, all they got is loadsa' shxtty novelty swords, and that's why they're dick-riding their local Yonkou. The Shichibukai run tings all day, because their powers're crazy as fxck. That's why Law got in and Kidd didn't, Kidd might be tough as nails (geddit?), but his power isn't half as amazing as Law's.
They're a whole loada' pirates (Kuro and surely many others) who don't want the Marine on their backs. There must be hundreds of them trying to gain the WG's favour, like Law and BB managed to do. Yes, they all sent the WG some scary proof, but so did a whole bunch of other pirates, whose powers did not cut the mustard.
They're not called the 'Seven Armed Seas' for nothing.