Yeah, some youtubers do follow through on playing demos. For example, Manlybadasshero plays lots of horror demos and then plays through meticulously when the full release is out. Markiplier played the demo of There is No Game a looong time ago and recently came back to the full release of the game.

I'm more interested in Death and Taxes since so many people played it, it'd be an interesting to see the full game. So I guess it may be good for a demo since it plants a seed for people to come back to. Its been a pretty long time since I seen it and I still remember it.