This looks like a reasonable use case.  That being said, I question whether the defaults should be attached directly to the template instance or whether they should be part of the substitution method.

FWIW, there already have a couple of other ways to do it:

>>> from string import Template
>>> s = Template("${user} made me a ${flavor} cake.")
>>> default = {"user":"Dennis"}
>>> s.substitute(default, flavor='vanilla')
'Dennis made me a vanilla cake.'
>>> s.substitute(default, user='Ken', flavor='vanilla')
'Ken made me a vanilla cake.'
 

>>> from collections import ChainMap
>>> s.substitute(ChainMap(dict(flavor='vanilla'), default))
'Dennis made me a vanilla cake.'