Within os and os.path, os.path.join() appears to be the
only function which fails to indicate a TypeError.

This was found inadvertently with:

def foo(*args):
    ...
    return os.path.join(args)   # note the missing "*"

Calling the function with:

foo('a','b')

returns:     ('a','b')

Other os & os.path functions return TypeError in this
situation.

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I'm -0 on changing this.  The code is already complex and
spread across multiple files.  It's absence of type checking
allows sting lookalikes to be used.  If there were a change,
it should probably refactor the function to use ''.join
instead of += logic.  That would detect the problem and
improve the algorithm at the same time.

+1 for duck typing.