One character per island is a concept which Dwilight had been testing for a long time. The results of that test have proven wildly positive. Interactions are better. Every single character can be expected to respond to people and have its own view on things. There are no placeholder characters. There is no "filling out" of realms. The characters you have are your realm. This allows you to know you're interacting directly with a character a player cares about each time.
Finally, the main benefit is that it removes blank slate characters. It seems you don't see this as a problem. Which is fine. My question would be: Do you dedicate he same amount of time and planning for both of your characters when they share a continent or do you have a primary and a support character? If you are like most people you have a primary and support character. I have done it myself. However, the evidence has shown that in the vast majority of cases (not all) this dynamic hurts the game instead of helps it.
