Mr. Meyers presents some very well-reasoned arguments against the everything-should-be-a-method mentality.
Note that the motivation for turning to string methods was that of migrating from strings to Unicode. Adding Unicode support to the strop C module would have caused very complicated code -- methods helped by enabling polymorphic code which is one of the great advantages of writing software for an interface rather than an implementation.
Of course. Meyers starts by saying that if it needs implementation details it must be a method. This is true (if only for efficiency reasons) for most string methods.
Note that functions can make very good use of methods and thus implement polymorphic functionality -- this is not about methods vs. functions it's about methods to enable polymorphic functions.
Meyers also says that if it needs to be virtual it needs to be a method. Polymorphism is roughly equivalent to virtual in this context, and this alone justifies the move to methods.
But join() is special: it is polymorphic in two arguments, and making it a method of the separator argument doesn't help.
--Guido van Rossum (home page: http://www.python.org/~guido/)