I think it's a niche market, I don't think it's stupid.
People inclined to pirate will find a way and screenshot-ing individual pages is tedious compared to the alternatives.
The reason I think it's niche is that I'm not convinced it works any better than samples for fiction. And samples are free, even a handful of pages here will cost - unless you speed-read.
But for textbooks or reference works or possibly even short story collections, I can see some demand.