there isn't a copy of the library in every executable because
the linker includes from the library only the functions the
program uses (directly or indirectly).   this is typically
smaller than the whole library.  there would be scope for some sharing
but the gain dynamically is typically small: not all the
executables in /386/bin are in use simultaneously.   after all, it's hard
to buy a machine with less than 64mbytes these days
and with plan 9 running rather a lot of that is free space, unless
you've got big memory applications such as ghostscript in which case
it's the application rather than the library that consumes the space.
i suppose if you'd got an enormous application library shared
by several applications in use at once you'd see more need
for library sharing.  in plan 9, that shared functionality might
be better represented as a file server (though not always).
dynamic linking might be of more interest (in Plan 9)
for selection amongst a set of implementations at run-time.