The whole point was to have a function (or class) that accumulates a
sequence and counts it. collections.defaultdict(lambda: 0) doesn't
achieve this on its own because it only knows how to handle sequences of
(key, value):

>>> d = collections.defaultdict(lambda: 0)
>>> d.update('aaabbac')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: dictionary update sequence element #0 has length 1; 2 is
required

The feature request here was mainly a request to provide an abbreviation
of a very common 4-line function.