I agree with this. I don't want content to be playable before release. It would only create tension between those who want to experience the game spoiler-free and those who want it done as quickly as possible.
That said, for things like job changes, I believe it would be OK. Just something like the media tour - one zone (not even a new zone, can be a repurposed old zone or one purpose-built for testing) with some striking dummies, a variety of mobs and maybe a repurposed dungeon and trial to test party stuff would be enough to try out how the jobs play and give feedback.