A Way to Handle "Speak With Dead"

I've got a way to handle the Speak With Dead spell narratively.  Now, in the Expert Set (Mentzer Edition) it says this:

Speak with the Dead Range: 10’ Duration: 1  round per level of the cleric Effect: Cleric may ask 3  questions By  means of this spell, a cleric may ask  3 questions of a deceased spirit if the body is within  range. A  cleric of  up to 7th  level may only contact spirits recently dead (up to 4 days). Clerics of level 8-14 have slightly more power  (up to 4  months  dead), level 15-20  even more (up  to 4 years dead). No time limits apply to clerics of  21st  level or greater.  The spirit will always  reply  in  a tongue  known  to the  cleric, but  can only offer knowledge of things up  to the time of its death.  If the spirit’s alignment  is  the same as the cleric’s, clear and brief answers will  be  given; however,  if  the  alignments differ,  the spirit may reply in riddles.

This brings up some interesting questions about the afterlife.  Most real-world religions teach that the body is simply a shell, and that the soul is what animates it.  Once the body is dead, the soul departs it.  In Egyptian mythology, however, preserving the body after death meant that the dearly departed would enjoy eternal life.  So why would a pseudo-medieval society believe that the body had anything to do with the soul after death?

Perhaps in our game we can say that the soul departs within a number of rounds, making it impossible to resurrect.  Now, after the soul goes to its final reward, there is a sort of "remnant" of a soul in the body, that makes it possible to contact the person in death.  Obviously the person will only know what they knew in life.  Again, because the soul remnant is tied to the body, you must be close to the body to do so.  In time, the remnant fades, and is harder and harder to commune with.  You may only pick up an obscure word or two, and that's it.  Eventually, this too is gone.  However, there is still some soul energy left in the body even after that.  This energy is what allows necromancers to summon an army of undead, using the soul energy left in the body to power them.  The more powerful ones may simply be recently dead, and are using a full power remnant as an energy source. 

Obviously, YMMV.