I can see the theory behind lightboxes, but in practice they're generally a bit of pain. Mostly because they're still relatively unusual and unexpected, so they break the flow of normal browsing.

But I'd put up with lightboxes everywhere if we could lose those awful on-hover link preview things stinkin' up the web.

If it conveniently replaces an unneccessary pageview I don't see what the big deal is. Everything for a purpose. They aren't universally bad.