The r-methods have already been implemented.  Here's the difference as of Python 3.4.2:

>>> set(dir(set)) - set(dir(collections.abc.MutableSet))
{'update', 'difference_update', 'symmetric_difference', 'intersection_update', 'intersection', 'issubset', 'issuperset', 'difference', 'copy', 'symmetric_difference_update', 'union'}

Guido intentionally omitted the named set-to-set operations in favor of the operator versions (__ior__, __lt__, etc).  It was not the intention of the ABC to implement the full API (see Guido's PEP 3119 for more of his rationale.

The copy() method was also omitted on purpose. It is a bit of a can-of-worms for abstract class to know everything it needs to copy the the concrete set.  This work is best left to the subclass which has the required knowledge.

I'm closing this one because I'm channeling Guido and thinking he really didn't want those methods as part of the ABC.