We should've gotten Oden's flashback way back in Act 1 when Kin'emon first recounted it.
By blanking out the story and only allowing us to see the Strawhat's reactions to the tale, while also making big mentions around the things that occurred in the flashback, Oda set our expectations too high. Expectations that weren't all met.
The placement in general was pretty awkward, too. It's not like Kin'emon and the others needed to motivate themselves then because things weren't as bleak as it seemed. The Alliance showed up mere moments, undercutting the need for the flashback.