User blog comment:Fintin/Benefits of Seperation/@comment-2117107-20100625175554

It was definitely necessary. Kuma ended up with a 296,000,000 bounty before becoming one of the Shichibukai. They all split up to avoid being captured, since they were up against an Admiral.

Like the manga said, the "Dark King" Rayleigh is 100 times stronger than the current rookies, including Luffy, and an Admiral is able to match him in battle. Considering how the 20 years that had passed since his prime may have affected his strength, it shows how powerful the Roger Pirates were when Gol D. Roger was announced as the 'Pirate King'.

The crew aren't able to defeat an Admiral, so the separation was necessary to strengthen their abilities and to be able to cope with situations when they are alone.

So yeah, the separation was necessary for them to get stronger and be able to cope with the level of power in the New World. It's inevitable that Luffy would have breached Impel Down and went to the battle at Marineford, though I reckon if the crew had went with him, someone would have been killed, so yeah, it would have been for the safety of the weaker crewmates, as well as to make them stronger.

Now, after reading Chapter 589, and with Luffy declaring he is too weak to be the Pirate King, he will do immense training on the island with probably Jinbei training him, to make himself stronger and also to keep himself safe, as I'm sure that with the situation Luffy put himself from punching a Noble, as well as helping with the nearly successful escape of Ace, his bounty will increase dramatically, as now Admirals will be after him, after finding out he was Dragon's son.