I can understand watching competitive gaming. I still find it boring, but it does make sense.
I also understand watching recordings of fast play-throughs. And even a 'lets play' that focuses on the game. I watch those sometimes when I am deciding if I want to buy the game.
But watching a casual game, while the guy talks about unrelated stuff, just does not compute for me.
I think Twitch got it right. The interaction with chat makes it a different experience altogether, regardless of what kind of game is being streamed.
Using playthroughs as a way to decide whether to buy a game or not is great. Gaming journalism/user reviews are often complete trash. One worrying thing though is that people putting these video on YouTube are paid to say good shit about the game without disclosing that they have had an incentive to do so.
I understand watching competitive gaming, because that involves something you might not necessarily experience by playing the game yourself. Watching casual gameplay, especially if it's recorded and not live, just seems odd to me.