As long as % formatting still works, it is in many ways more convenient that format. '"Foo: %s" % bar' is a lot shorter to write than '"Foo: {}".format(bar)'. And % formatting is still built into logger, so if you use logger heavily you will probably be used to it.
While the format() mini-language is nicer in a lot of ways, the convenience of % and its ubiquity mean that I generally use it more often. The main time when I reach for format() is when I need more detailed control of column widths and the like, which you can do with things like "{var:{width}}".format(var=var, width=width)
Personally I dislike % formatting even if it's a few characters shorter to type. .format is superior in every other way and has consistent-with-everything-else method call syntax.
Edit: I think it's largely the special flower overloading of the % operator that bugs me the most. If % was spelled .sprintf I'd like it more.
While the format() mini-language is nicer in a lot of ways, the convenience of % and its ubiquity mean that I generally use it more often. The main time when I reach for format() is when I need more detailed control of column widths and the like, which you can do with things like "{var:{width}}".format(var=var, width=width)