Arcanist can't reliably heal anything past level 25. White Mage can't DD nearly as effectively as a real DD job, even with Cleric Stance.

The fact that the game limits you to your "intended" role is probably better than the alternative, as you say. Who wants to wait an hour to get into an instance, only to find that their tank is an Arcanist with Topaz out, their healer is another Arcanist, and both of their DDs are conjurers?

As it is, you can get around the restriction by putting together your own party. I've run plenty of low-level groups without a real tank or a real healer. We one-heal/one-tank Praetorium every single time. As long as you put together your full party (and thus not subjecting any randomly-selected players to your weird group setup) then you can do whatever you want!