Agree with Cow's comments on it depending on the route. When I used to fly YYZ-GRU with a client that would spring for J, there were many times where we had to alter travel dates because the cabin was completely sold out, in some cases two weeks before the travel date. I don't believe there were too many non-revenue upgrades on those flights, not when I sometimes couldn't even buy a seat and was paying full pop. At the time, return J fares were anywhere from six to 10 grand, depending when you went and how far out you could book.
Deadheading employees remain another matter altogether...