@Elric:
don't think so.One strong pirate wouldn't help since he can't cover all of the grandline so they decided it had the be 7 of them.
Except that makes no sense. Seven is an arbitrary number so do not bring that into the discussion; regarding strength, the Shichibukai are a NUISANCE the the World Government, and through their special contracts they become less of a nuisance. However, if de Flamingo messing with the Marines is any indicator, the World Government would likely wish they were rid of the Shichibukai if there wasn't a need for them, as they've got the Marines, Buster Call, and the Supreme Admirals to clean house for them. Why need outside help? If the World Government could kill the seven Shichibukai, they probably would in any way possible; since they can't, the secondary option is to make them allies.
@Elric:
You can't let one kill another since you already got the strongest available and every possible replacement would lower the average power of the shichibukai and therefore their overall use.
Despite what some pirates might say, the Shichibukai are not government dogs. They're legal pirates, which means they can do whatever the fudge they want without getting pestered by the Marines (like Arlong); this includes "liberation" like what de Flamingo was doing or pirate hunting like Mihawk.
Since they're not fully under the control of the World Government, I'd imagine the WG would rather be rid of them than have them as allies.
@Elric:
so they are the strongest available and can't be replaced just like that so even if one could kill another it won't be allowed and therefore their individual strengths could very well be quite different.
I'd imagine the Shichibukai to not be the "strongest available", they're the strongest/most reknown/fearsome folks that were willing to put up with the World Government for some reason. If the government couldn't kill any of the individual Shichibukai and approached said Shichibukai asking for a deal, whereby the government wouldn't "pester" them in exchange for loot earned by the Shichibukai, WHY SHOULD THE SHICHIBUKAI PIRATE COMPLY? It's like for one guy in a chess game who is about to lose make a deal with the potential winner to call the game a draw; why do that? Unless the Shichibukai have an alterior motive, there's no reason to join up with the World Government.
Croc's reason was likely to get contacts, funds, and resources for establishing the Ideal Nation. de Flamingo's is probably connected to his belief in the coming of the "New Age of Piracy". I can't say anything about Kuma or Jimbei.
@Octogon:
**We havn't seen anyone stronger than Lucchi show of more than 5% of their ability. Smoker, Ace, Mihawk and AoKiji are the ONLY characters above Lucchi that have shown any ability. Mihawk moved faster than the eye three times, 2 times with his hands and the other going from his little boat to Krieg's. We saw Smoker appear behind Luffy before he could notice, he even commented on how slow Luffy was I believe. And AoKiji speed feat was grabbing and lifting Sanji and Zoro before they even noticed.
We also KNOW the high-tier OP chracters can move faster than Lucchi's soru. It's common sense. And it's very, very, very, very, very likley they can move much faster than lighting.**
See, you were making sense until that car wreck of a second paragraph reared it's ugly head. If Mihawk moved "faster than the eye can see", he's roughly equal to Soru in speed, not faster than it. Smoker, like Croc, can fly, and sometimes when those two fly, they manipulate air to propel themselves at stuff like rockets. Aokiji's speed may or may not be related to his Devil Fruit.
And, again, I don't care how fast Lucci is, there's no way he could move faster than lightning unless he was a Saiyan. Even in a manga where people can kick with the same force of trucks, moving faster than the speed of light has yet to be accomplished. The concept of people saying they see individuals "moving faster than the eye can see" is just a writing device to put extreme speed into context with normal speed; it's not a literal measurement of speed at all.