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Being forced to number the arguments when using positional arguments in
a format string is difficult to maintain. Adding an argument to the
format string either requires renumbering all subsequent arguments, or
using an out of sequence number such that the order of the format()
arguments no longer matches the order of the "{...}" arguments. Making
the integer optional would solve this. Thus, "{}" would be like the old
"%s" where it was far easier to add %s arguments in the middle of the
format string.
Python 3.0a3

Omitting the ordinals from {} format units will present a problem with
internationalization because the arguments may need to be rendered in
different order in different languages. This is a frequent problem with
the % formats and it is a good thing that the new format requires
explicit enumeration. If your argument list grows to the big enough
size that renumbering presents a problem, you should consider using
named rather than positional arguments.
This type of changes should be discussed on python-ideas list first.

This was proposed on python-ideas, discussed, approved, and implemented
for 2.7 and 3.1.
Note that although this wasn't discussed, the internationalization issue
is answered by the fact that when internationalizing, you are rewriting
the string. At that time if you need a different order of the
subsitutions you can specify it, so requiring non-default numbering in
the original string doesn't really buy you anything for
internationalization.