Why do multi-methods allow open access to parameters? It allows efficient handling, like C++ friends, usually by allowing representation details of more than one object to be exposed.
[Kim 89, ch 4, pp70-71 (D. Moon)] for an alternative explanation. While open access can be useful in some cases, it typically isn’t recommended as a general OO practice (see section 1.15, C+W’s requirement 1 for OO languages and Section 1.2 on Encapsulation) and also violates subtype polymorphism, because only subclass polymorphism is based on representation and not type. Polymorphic languages can be statically typed to provide strong type checking, efficiency, and to support a static programming idiom, but require restrictions in many cases, such as requiring overriding methods to have identical signatures with the methods they substitute (as in C++) or allowing covariant parameters but limiting base class usage (as in Eiffel). If these restrictions are dropped, multiple polymorphism results. Thus a single overridable function declared in a base class may have several functions overriding it in a derived class differentiated only by their formal argument types.
