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aesthetics, NIL and T

Although the discussion would not lead one to believe this, I suspect
that at least some of the motivation is based on implementation
strategy.  That is, if NIL is an atom, and can have a property list,
then it cannot (perhaps) be stored in "location 0" of read-only memory
(or whatever hack was used to make (cdr nil) = nil).
This kind of consideration (though maybe not exactly this), would eventually
come to the surface, and unless people face up to questions like how
much does it really cost in implementation inconvenience and run-time
efficiency, we are whistling in the dark .  I reject the argument that
has been advanced
that it costs nothing in some dialects,
unless other strategies for the same machine are compared.  In
some sense, you could say that "bignum arithmetic" costs nothing in
certain lisps  "because it is done all the time anyway"! Ditto for
some kinds of debugging info.