I think this is a bug.  In GNU Emacs 21.3.1 (i386-pc-solaris2.7, X toolkit),
I did this in the *scratch* buffer:

(add-hook 'pre-command-hook
           (lambda () (message "pre-command-hook: %s, %s"
                               last-command this-command)))

And then typed:

M-x list-buffers RET
C-x b

Which generated this in the *Messages* buffer:

pre-command-hook: eval-print-last-sexp, execute-extended-command
pre-command-hook: execute-extended-command, self-insert-command
pre-command-hook: self-insert-command, self-insert-command [11 times]
pre-command-hook: self-insert-command, minibuffer-complete-and-exit
pre-command-hook: list-buffers, switch-to-buffer

Note that there is no "minibuffer-complete-and-exit, list-buffers" entry.

Fexecute_extended_command sets Vthis_command (but not Vlast_command)
before calling the Lisp function (list-buffers in this example) via
Fcommand_execute, but neither one of them runs the pre-command-hook
(or post-command-hook) Lisp functions.

-- 
Kevin Rodgers