Before we get too into it, I should point out that it is actually a different "worldspace", not cell, reffering to how cities and interiors were in OB.
I suspect cities are now open, but interiors will be seperate worldspaces. We simply don't have the power to run all that stuff at once, especially with items in interiors with physics.
I didn't mean prison cells, they actually are referred to as "cells" at least on UESP. Well if we don't have the power how did Risen do it? Not that Im insulting anything or saying I like Risen better, but there were zero loading times and no closed areas in that entire game, not that I expect that from Skyrim, but it would certainly feel more immersive if I could see the outside world through holes in the ground when Im in a dungeon and other things like that