"guix shell" is for making packages available in the
environment. Currently, "guix shell -- foobar" does not make any
packages available -- it's effectively a no-op except for setting
GUIX_ENVIRONMENT.

True, though you could always have scripts that read:

  guix shell $packages -- whatever

and that will suddenly behave differently if $packages expands to an
empty string.  Tricky!

Right, I didn't think of such uses.

Greetings,
Maxime.