While I personally prefer %-style string formatting, I refrained from trying to implement it, because having both str.format and str % tuple seemed like redundancy for small Python implementation but:

1. http://legacy.python.org/dev/peps/pep-0461/ reinforces %-formatting as active, supported Python feature, which once again offers more functionality than str.format.
2. There's existing code which uses %, so if we want to run it, we need to support % (I hit that when trying to run tests from CPython testsuite - rewriting them besides being monkey job leads to inability to report that we're able to run unmodifed CPython testsuite).