[Speedz]

Those are 2 great points. I did think of the epic battle scenario.
I think if it was doable to implement this, maybe have it set that each zone can have multiple instances based on group and raid allows only.

Maybe even a stretch to not change the level of currently agro'd mobs.

The other point, I would not be against moving this to feature requests if that is more applicable.

As far as why?

Well to see if it can happen (the challenge).
Also open more ideas to custom worlds and game design.
A few examples could be a diablo type structure and the fun of exploring previously high level zones as you level up. We all know the grind of the same old zones at any given level. Even if the server is customized to heck and back, it can get boring being locked into the same set of zones per level.

Could make for interesting scaling for raids. For high end raids could also add a modifier based off average level and number of people in the raid. Then scale loot to expand on this as well.

Last example could be that the "instance" world can be limited to zones the admin sets. So then they could set server events that cover zones previously made and populated with more options available.