If Oda started the flashback after Ace getting magma-fisted by Akainu in chapter 574, it would be so painful to read. And we had the 100 page punch when Luffy beat the shit out of Wapol.
And, the point of this flashback is for Luffy to realize he is damn weak and need to train much more if he doesn't want to lose any important one again in the wild and terrific battlefield of a complelely different level-New World, which, as I believe, shall not be achieved by a couple of years of training. You can say,with it giving Luffy the reason to stay with Jinbei and Hancock longer to power-up, the flashback is Oda's preparation for a timeskip. Maybe a long one that lasts one or two years or even more.
His crew, after witnessing the level of this world's top dogs through tv (lol) and other medias, may hold up to train at their current place before regrouping.
All screams for a timeskip.