It's extremely obvious that if Shanks, Whitebeard, the other two Yonkou, and Rayleigh are all in the fight that it would be a rather large problem for the World Government, you'd have to be stupid to say that it wouldn't be. Garp, who is arguably one of the most experienced Marines in the entire fleet, said that just TWO legends at once would be something that they couldn't "Afford", add four more legends and say it's trivial? Lol you can't do that. I'm with Ivotas on this. Hell, add Dragon on top of that? The world government is confident, but so far, you have to realize, they can't even stop Dragon, let alone the Yonkou. That's right, you think they haven't sent Kuma to deal with Dragon, but they have to deal with Luffy? BS, Dragon's force, for whatever reason, is thus far untouchable by any power the World Government has. Why do you really think they need Poseidon? To deal with the pirates? Yes, the pirates are a pest, but that is just a cover, Dragon is the greatest threat and the real reason they need the weapon. But you guys really think that the four pirate emperors being added on top of that problem wouldn't be a big deal?
I find it suspicious that they're having the Shichibukai deal with Whitebeard, it tells me that they await a greater threat, otherwise, why would Sengoku be against Kizaru leaving, if the admirals aren't even the one's dealing with Whitebeard in the first place?
It's canon that Dragon is the greatest threat to the World Government, it's obvious that to the greatest threat goes the greatest resources. In my opinion, from what we've seen of the Shichibukai and the fact that Luffy has defeated every one he's encountered, the four Yonkou are certainly stronger than the Shichibukai. If the Shichibukai can't hold the line and Dragon involves himself, they're in trouble.
In other words, friends, the real reason that Garp fears multiple legend interference is Dragon. He fears how his son will take advantage of the situation, that's why the Marine's can't afford to take huge losses even if the win. In other words, if the World Government WINS, but takes large losses, Dragon, Garp's son, comes into action. This is the real risk.****