It's going to depend on the specialty. In one of these other threads, it was discussed regarding peds residencies, and saying how having a letter from outside of peds is okay, even encouraged by some PDs. However, the surgery attendings chimed in saying that it would be unusual for someone applying to surgery to have an IM or FM letter.
I think if you're applying to IM, and you have a good relationship with your FM preceptor, then there would be nothing wrong with sending that letter, since the fields overlap.