Rationale (July, 2009):

The LWG decided not to add reference qualifiers in the library, which reduces the motivation for making this change to implicit assignment operators.

For increased regularity between built-in types and class types, the copy assignment operator can be qualified with &, preventing assignment to an rvalue. The LWG is making that change in the Standard Library. It would seem a good idea to make a similar change, where possible, in the specification of implicitly-declared assignment operators. This would be the case when all subobjects of class type have a non-deleted copy assignment operator that is &-qualified.