Phasing isn't quite that regimented. My wife and I played together, and it really depended on the quest we were on. Sometimes we were in our own phases, but most of the time were together (but we weren't generally out of sync on the quest).
I personally love the phasing system -- you see real, and seemingly permanent changes in the world, and I don't recall seeing that before in ESO's fashion. I always hated how most MMO's rely on the suspension of disbelief when it comes to the game world...it doesn't matter what you do in the game, nothing really changes. Like LOTRO for example, during the years that I played it, it didn't matter that you went through the Angmar and Moria campaigns, you could go back to the Shire and it was like nothing ever happened.
In ESO, that's not the case. As you quest, the world changes around you (phasing causes you to see things differently than other players). So, as you quest, you might see a pristine town get partially destroyed, you then continue to see the town in ruins, but other players who are earlier in the quest/story don't see it that way. I think it's great for immersion.