> In fact, I have a hard time envisioning any program where the cost of
> printf() is noticeable at all.
I have one. I'm doing data-capture and pre-processing on a relatively slow system (mini-itx, to keep the heat down so as not to cause optical turbulence in the lab). We acquire data at about 1 MHz on 4 channels, and need to do some simple preprocessing (offset and scale) then save to file. The file is a 4-column text-file as asciihex eg "0x1234 0x4567 0x3432 0x6666\n", i.e format string is "0x%x\t0x%x\t0x%x\t0x%x\n". So the printf cost is definitely not negligible, when we call it 1 million times a second.
(Yes, we could optimise further and keep the data in raw binary form, but it's not strictly necessary yet, would prevent us adding headers, and would make for a non-human-friendly file format. But I'd certainly welcome a fast printf for the simple cases)