Will it stick to the default image for those of us that don't use bar-codes?
It sticks to the default image for everyone. The picture that illustrates a movie is the same for everyone, it has always been like that, and it remains like that.
The novelty with version 3 is that your owned videos can now have a cover image. If you don't give a barcode to your owned video, your video won't have any cover. If you give it the barcode of an existing video, your video will have the cover of that existing video.
Even if you don't have videos with barcodes, you'll probably use the barcode field, just to have covers (it'll be possible to copy & paste, making it easier to take the barcode from an existing video).
To sum it up, version 3 displays the default poster of the movie (like now), and it also displays, below it, the cover of your owned video (that's new).