You might be interested in this thread from my multimethod library: https://bitbucket.org/coady/multimethod/issues/2/.  It was a feature request for multiple dispatch to work on instance methods.  I implemented a variation where the initial self argument still contributed to the dispatch.

I think the same reasoning would apply in the single dispatch case:  that it’s not obvious the “dispatch” would occur once on the self argument to find the method, and then again on the “first” argument.  Especially since Py3 did away with bound methods, so you can call a method through its class and explicitly supply self as the first argument.