The point is that if you're not enjoying the journey, you skip to the end and take out the "I need to get to the destination" motivation for finishing. I do this all the time: if the book is getting tedious, I prevent myself from compulsively finishing it by "giving away the end" to myself.
I do the opposite. If I'm finding the novel a slog, I'll read the Wikipedia synopsis. If the payoff seems worth it, I'll keep going - I think that most novels where a "twist" isn't the only selling point are worth reading even if you know the plot.
(And if the payoff doesn't, I drop the book. That's what I did with Mieville's Embassytown at the 50% mark.)
