There is a real strong purpose behind Zaku's flashback...it's to show that Zaku isn't necessarily an Evil character. He had hopes and dreams (much like naruto), but his encounter was with Oro brought him on the wrong path. that's what i like about the series...with the exception of a few, most "evil" or "bad" characters have a reason for being the way they are.

it's similar to gaara and haku's flashbacks, as well...yet I don't hear people calling kishimoto an idiot for doing those stories.