Why is `C-M-x' only for top-level defuns?

[Stephen J. Turnbull]

Drew Adams writes:

 > Why not let `C-M-x' re-evaluate a "defun" (defcustom, defface,
 > etc.) that is not necessarily at top level?  E.g., with point on,
 > say, `defface' in this sexp, why shouldn't `C-M-x' redefine the
 > face?

I don't see any reason in the `when' you're talking about, but in many
cases such forms will refer to let-bound variables and the like, and
the results there could be rather confusing.