@AMS:
Maybe you enjoy them on a deeper level but you cannot make a case that Wapol's, Gedatsu's, CP9's, or most of the barouqe works one are important to understanding or appreciating the ongoing story of one piece.
World building.
Character building.
Canon source material.
Possible later signifigance?
Endings for characters we spent dozens of episodes watching.
They're all great reasons the material should be adapted. Theres not a single good reason they shouldn't.
@AMS:
And stuff like the Hachi back story didn't help anyone understand the arc better.
It showed how he and Caimie and Papug met, and meant the second we saw Caimie, we knew Hachi would be back. Its a level you don't get without the background info.
It was just a nice little treat
Exactly. A canon treat, at that.
Oda just thought of something cool to do with the covers, but overall I reckon most of them are about as important to the story as pandaman.
Except when they're incredibly important, like learning of the lost history, Enel getting a moon army, CP9 becoing pirates and swearing to come back, Ax Hand Morgan being set on the loose again, Wapol creating Wapolmetal which was used during the Marineford arc, seeing how Buggy met Alvida, why Django is suddenly randomly in the marines….
(something that COMPLRTELY confused me when the anime got there. PLUS he was friends with fullbody? It was completely random and made NO sense at all... except oh wait, there actually was a story that told all about it. it wasn't a random drop in with no explanation in the manga.)
Its part of the world and until the end we don't know what will be important and payoff, what is foreshadowing, but considering how most of the coverstories HAVE come back to deal with the main story? It's important. Hell, the last five pages of Enel's coverstory may well be more important than anything else in the last 60 volumes of material.
Certainly much more than Hancock filler where she has a dream sequence about feeding Luffy for 20 minutes that REALLY has no bearing on anything..