My understanding is that when a mortal dies in the planes, your spirit ends up in another plane and is trapped there depending on how you lived your mortal life.
Mortal adventurers can teleport between planes using portals. But, Spirits cannot use the same portals... right?
So when a evil man dies and is sent to the lower planes and enlisted into the bloodwars for eternity, do the baatezu or tanarri give him a new physical body? or does he fight as a spirit?
There are no spirits or ghosts or other such beings on the Outer Planes. There's no ethereal to support them. Certain things are incorporeal, granted, but they aren't going into the ethereal. So, when a basher dies, his soul goes, via astral conduits, to the plane that most closely matched his alignment (and, according to some sources, straight into Asmodeus' mouth if he didn't believe in anything). There, he is either absorbed into the plane or becomes a petitioner. Petitioners are solid beings that are unique to their specific planes and usually don't leave them for fear of complete oblivion. They serve as the miscelleneous and unimportant NPC's in most PS adventures, kind of like the average farmers and low level guards on the prime.