caching in load-time-value and defmacro

[<don...@is...>]

 >    2. Re: CLISP support for a new portable FFI library (Hoehle, Joerg-Cyril)

 > LOLAD-TIME-VALUE is not so nice in the interpreter, because there
 > it's basically IDENTITY and thus re-evaluated each time in a
 > loop. In the interpreter, one would wish for Scheme's (FORCE
 > (LOAD-TIME-VALUE (DELAY ...))) to be built into load-time-value
 > instead of the current behaviour.
I don't know what scheme does but I had assumed that load time value 
was caching.  The same goes for macros.  I see now that this is not
the case.  Is there some reason these do not execute only once?