Nah, that's not a shared experience. If I choose a player character, that should be the one everyone sees. Especially given how ludicrously different the modern models are than the older ones. If they had tried to port over everything perfectly instead of create a new vision, this line of reasoning might be kinda acceptable.
New models is not vanilla. Period.
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I would assume the main benefit is that you are keeping only the stuff you need, at the cost of access time. If you make a single data structure that needs to remember everything, you end up with, like they showed, the fact that fireball has three auras, one of which is empty, and then it has three aura data points, one of which is periodic damage and two of which are nothing, and then it has three damage data points, two of which are populated and one of which is empty. In the other model, fireball has a reference to its damage and a reference to its periodic thing and then some termination concept, and then each of the damage and its dot has that value and nothing else. They probably made the change because they wanted to be able to decrease the memory weight of the database at the cost of a couple more references, and didn't keep support for the older model.
I feel what they should be doing is adding in the support for the older model of thing so they can use the old data in place, because that's a truer emulation of the server side. But that's clearly not the direction they are going. This way should, in theory, work just as well, at higher initial effort cost.