In order to get people to even want to job swap in a dungeon they would have to create a need to do so. Which would require them to create a need to do so. Which would leave certain jobs behind.
While SE obviously isn't perfect at avoiding this as it is, this goes even more so against the balance they've wanted from the start. It was/is a neat thought but the reality is that implementation is too hard if not impossible to pull off with the balance they want for each job.
