I've bene full flights, and on nearly empty flights across the Atlantic, so I wouldn't agree they are all empty.

Personally, I have never been comfortable stretching out across coach seate, even if I was the only one in coach. If it were me, and I wanted to arrive fresh and wide awake, then I would go for the upgrade (say if I was flying overnight to a morning meeting). If however, I was flying all day to get in and grab a hotel for the next day I wouldn't normally use the upgrade.

Given a choice though, a bad seat in business is way better then any coach seat, even if coach is empty.