"If your page scrolls, you should have some indication that your page scrolls."
That's my approach. I don't think scrollbars are entirely necessary (and in a slide-to-slide webpage like this, having a scrollbar wouldn't really work since it'd break the UI), but having some indicator that you can scroll is necessary. It doesn't necessarily have to be a scrollbar though. Having content peeking out from beneath the fold is often enough to let the user know, "Hey! There's stuff down here! Scroll and check it out!"
That's my approach. I don't think scrollbars are entirely necessary (and in a slide-to-slide webpage like this, having a scrollbar wouldn't really work since it'd break the UI), but having some indicator that you can scroll is necessary. It doesn't necessarily have to be a scrollbar though. Having content peeking out from beneath the fold is often enough to let the user know, "Hey! There's stuff down here! Scroll and check it out!"