Depends on the game. Many games are only remarkable for their combination of story and aesthetics, with the minute-to-minute play being fairly mundane. In that case I can see a bunch of people just watching a stream and not bothering to play the game themselves.
How many of them would have been so pushed as to play the game for themselves anyway... an open question.
Games like "The Last of Us" and "Uncharted 4" definitely seem more like audience-participation movies. The fight scenes might be fun to play through, but a lot of the good stuff comes from the scenery, the story, and the acting - both voice and capture.
I watched Let's Plays of both because I wanted to see the story, and because the players were entertaining. I don't think I'd have enough time or patience to play through either game, but I definitely got value out of the game itself without having played it.
It does seem like a gray zone - I experienced content without paying for it, but I didn't experience it the way it was truly meant to be. Did I come by it unfairly?
How many of them would have been so pushed as to play the game for themselves anyway... an open question.