Imagine you're flying a space-capable plane, fighting enemies on the ground. (that's obviously one cell, managed by one server).
Then another aircraft starts chasing you and you decide to turn this into a space battle, knowing your faction has a much better advantage up there than down there (they moved a space battle station above the position they were attacking, but the ground forces of the enemy are still alive and kicking, for example). (the space will obviously be in another cell because it's so incredibly far, and contains it's own lot of objects and stuff).
Now in most games you'd climb and climb, and see a loading screen, then climb again.
That would make no sense at all from the perception of the aircraft in pursuit, and would break a lot of the immersion.
We need a solution without any loading screen or visible server switching to make this just awesome (imagine an aircraft chase from the ground into space, a space battle, and then the chase reverses as the hunter becomes the hunted, and flees down to the ground -- no interruptions, no loading screens, only amazing immersion).
For the server-side, I think the solution would be slightly overlapping cells, and silent connection to the next cell's server (servers actually because if you aim for the top corner of a cube you will be in a 4-cell overlap) at that point, tracking the aircraft and its projectiles in both grids.
The client would have a proper solution for dynamic loading of resources (a bit like a moving window concept) and would reconcile the inputs from multiple servers, discarding duplicate input (like when four cells track the same missile, only one collision will be recorded and the rest is safely discarded) and making a seamless transition between two cells, effectively creating an infinite immersion, no 1000m flight ceiling, no "go back to the combat zone", no crap.
If you want to take the 60 miles detour to attack from a better angle, suit yourself.
If you want to go from the planet to its moon in your little aircraft, suit yourself.