I don't think it'd work this way, especially when someone's already playing something they don't want to. Your suggestion would make the issue of bad tanks/healers a bit worse. They'll just do enough to get by (if anything at all beyond keeping the enemies on them and some people alive, maybe not even that) until they get to the job they actually want to play. There's so many ways to level and you better believe that they will take the easiest possible route to get to where they actually want to go. Either way it's definitely not a good idea. Some jobs are already level locked. That's all the work that they should have to put into unlocking them.
