I think a January clerkship is worth it. Of course, many programs will have students they already like and have a short list going in to interviews. It might be hard to convince them that you are better. But if you are, there is no reason they wouldn't rank you high since the submissions aren't due till the end of January (unless that has changed??). One of the residents a year above me didn't have an official rotation, interviewed in January and it went well and so he ended up spending a few weeks at the program immediately after interviews. He was ranked and matched. And this is a program that typically doesn't fall past the 4th ranked student, so it's competitive.
Obviously rotating in February or March (after interviews) is completely pointless. And there are some programs who notoriously have tried to get students to agree to rank them in December and skip interviews all together, so those places would be a waste in January. Not sure how prevalent that still is though..
