Having held coho and chinook jacks in my own hands, and squeezed milt out of them into a bowl of eggs from an adult female I can tell you that jacks do in fact spawn.
Just as Japander said, it's a way for nature to keep the gene pool diverse over the years. Also the more jacks there are in a year will give you an indication of what the return of adults will be next year. Lots of jacks, lots of adults the next year.
Some think that Jacks may not spawn wiht one partner, but rather they sneak up beside a couple of adults and release their milt at the same time. I don't know what the success rate of jacks spawning in the wild is, but do know they spawn.