> Other programmers write libraries, functionality that is meant
  > to be used by other programmers to build their code upon.
  > In this case, you rarely know the context in which a user of
  > your library will use it, therefore you
  > cannot really make any decision about types.

Uh ? Even for a library you certainly can make decisions about types, document them and enforce them. Who wants to accept unspecified data types ?