As Tanuki has already said, dating someone with the intention of making another person jelous is a horrible, horrible thing to do. People who do this aren't mature enough to deserve a relationship in the first place.
I would consider that sort of behavior to be beneath Ginny. Yes, there are a lot of people who do it. But they are jerks.
I think people put too much weight on Ginny's initial feelings for Harry. He was, twelve, she was eleven it's not like either of them were in any position to be knowing who they want to be with for the long haul. It was a schoolgirl crush and that's about it. Over time she learned to be less nervous around him, got over her crush (for the most part anyhow) and dated other guys.
I think that Ginny has always felt something for Harry on a subconscious level. I don't think that she made any concious decisions on who she would date based on that though. I don't like the idea of her conciously nursing a crush on Harry whilst she's dating Dean.
but you will make do with the next best thing.
I like the idea that she was genuinly interested in Dean to begin with, but then things just didn't work out. I hope she wasn't treating him as "the next best thing". I've been "the next best thing" before, and believe me, it's not nice at all. I think that people should either make sure that their feelings for one person are gone before looking for the next person, or make it known to the other person from the beginning what's what.