I really can't understand this criticism of Oda's work. If anything the Marineford Arc was amazing and heart-wrenching. The storytelling is perfect and the events transpiring are explosive, well timed and unpredictable. Why would you want for Luffy to acquire One Piece when in my eyes he already has it. The best treasure a pirate can get is his ship, reliable and amicable crew and bunch of exciting adventures. At least in the world of OP. And unlike other anime (which can also be very good!), they actually face tragedies, love, hate, respect, the ever present discrimination and oligarchy through the World Government.
I don't want them to find One Piece because that would mean the end of a really great show. It was amazing to watch a battle where the main protagonist is actually one of the weakest there, but he can still surprise and give everything he has to accomplish his goal.
I could go on for pages in describing what I love abut this show but that is not the point.
Are there flaws? Of course and I wouldn't want it any other way. Since there are no perfect things. But it is still excellent show :).Now, back to the question of the flashback with Ace. My believe is that it serves the purpose of introducing Sabo and who knows maybe One Piece is actually the chance to bring back those whom you love.
P.S. My apologies for my atrocious English. Not everything is in spelling correctly. :)
Speaking of the Marine Arc, does anyone find it odd or ridiculous that there were little or no causality besides the nameless soldiers, Ace and Whitebeard in a war that huge?