The intended use for str.encode is for same-type transcoding, like this:

I was unaware of the seemingly useless behavior you quote.

>>> 'abc'.encode('base64')
'YWJj\n'
>>> 'YWJj\n'.decode('base64')
'abc'

Here is a similar use for unicode.decode.

>>> u'abc'.encode('base64')
'YWJj\n'
>>> u'YWJj\n'.decode('base64')
'abc'

Any doc change should make the intended use clear if not already.

(Note that the above give lookup errors in 3.x
>>> 'abc'.encode('base64')
...
LookupError: 'base64' is not a text encoding; use codecs.encode() to handle arbitrary codecs)