I thought that Ginny was actually used pretty effectively in Book 2. As well as being an integral part of the plot, I think that she was symbolic of all the little wizard girls out there who had a crush on Harry.. Just when I was eight all the little girls had crushes on the New Kids on the Block and when I was fifteen all the teenyboppers had crushes on Leo DiCaprio and went to see Titanic ten times. I think that the actually Harry and Ginny relationship was thrown into the end of the HBP because Harry must have a love interest... It made very little sense and unlike Ron/ Hermione (which was brewing for seven books), seemed very unbelievable.
Yeah, I would've liked to have seen more of her in the film version. Had I creative control, I would've rewritten the film to tell the story in a better way for the medium rather than following the book so closely. The story should've followed Ginny a little more so we could've seen what was happening to her. It wouldn't really have been giving much away -- most knew what was coming. And only the people who had read the book in advance noticed Lucius dropping the diary into Ginny's cauldron. (There wasn't even a flashback when Harry realized it.)