Re:I see some issues here... (Score 1)

Now, with video-on-demand (and live streams), the whole Tit-For-Tat system no longer works. In this situation, peers must obtain pieces in order for playback.

Not completly true. With video-on-demand it is enough that you get the occasional later piece that you can trade for more common earlier pieces. Of course, as clients are prioritizing the earlier pieces the tit-for-tat will be less efficent. Not useless though.

Yes, you're right. In my comment I simplified the situation. A peer can indeed also fetch later pieces while downloading the earlier pieces in order.