Introducing Oars Junior at first seems like a way to hype up the Warlords while giving Ace some easy sympathy points, but much like Aokiji freezing the bay it has far reaching implications for the rest of the war.
There's been a lot of discussion here about the (lack of) pacing during the post time skip chapters, and in addition to there being too many characters in general, I feel like this is the other thing that's missing. Throughout Marineford (as well as Water 7 which also does this quite well) events rarely happen in service of just one thing. It's no surprise that those are two of the highest regarded arcs in the entire series.
And in fairness to Oda, this sort of multilayered writing is extremely difficult to pull off during weekly serialization. It's the sign of having a clear vision for the entire arc, which is equally amazing given how much Oda makes up by the seat of his pants.