Maybe this is an extreme point of view - but don't you think it's inconsistent that you can't have a parent slot 'implemented' as a method? OK - so it might be a bit slow - you'd have to call the parent slot method(s) for an object for every message it received.
Given the obvious implementation difficulties, a pragmatic answer could be to ask for some examples demonstrating the usefulness of methods-as-parents. (Implementation is difficult because it's hard to "constant-fold" a method execution and thus to inline a send whose lookup goes through a "method parent".)
But besides the implementation difficulties there are some semantic problems: in what context is the method evaluated (i.e., what is "self")? And what happens if the method sends a self message? The resulting (potential) infinite recursion would seem to limit the usefulness of methods-as-parents (you could only send messages accessing slots in the object itself or in descendants).
On the other hand, they would be a wonderful tool for an annual Obfuscated Self Contest. :-)