@i:
Ace's death was the stepping stone for the TS. If Luffy actually saved Ace then he would never of had a reason to make himself stronger because he was already strong enough to protect his loved ones. Killing Ace showed Luffy that there are things he can't control and people he can't hang with yet. It terrified him to realize that he wasn't strong enough to protect his own brother, combine that with Kuma separating the SH's at SA right in front of his eyes and you've got one seriously scared strawhat (sorry, I had to). Not to mention the combined deaths of Ace and Whitebeard are also more of a symbol in the world of OP now. Two of the most renown pirates in the world were just publicly killed by the Marines, people felt safe for all of 10 minutes until the next batch of pirates decided to start ransacking towns in search/celebration of One Piece.
I'm not entirely sure this is the place to discuss this, but I don't really agree with you.
It's true I've been out of the OP world for a while and I had a lot to catch up before starting to debate the story again, and even though I think the reasons you pointed out could be the real ones, they still don't sound good to me. Sorry.
Luffy had to get strong, I think he allready realized that, when Kuma managed to get the crew away from each other, so, even though Ace's death was really important to this matter, it's not fully the reason why.
Whitebeard's death was more than enough to cast the whole world off balance, it really wasn't necessary to add Ace's death to this.
So, in the end, I think Ace's death served only for the purpose of Luffy coming alive from a battle where winning was impossible for him. Ace fulfiled his dutty as an older brother (even though not biological), he protected Luffy. And I think that that, is more than enough for a reason.
Getting to the poing (the new chapter), this was great. Had me crying when I remembered Nami's past again and it kind of had me feeling really bad for all of the Fishman, why can't they have peace with the humans? I want to see what's the flashback all about and, somehow, I'm eager to see the end Oda-sensei will give this arc.