who's offering an off-match?
Unless you're an IMG and/or worried about not matching, its hard for me to see the advantage of taking an offer outside the match...
As an IMG candidate, I probably know more about the prematch situation than many on this forum.
--All the independent applicants are eligible to take an out of match offer. Independent here means one that's not sponsored by an LCME accredited medical school. Copying from the NRMP website, independent applicants include the following:
(a) A physician who is a graduate of an LCME-accredited medical school;
(b) A student enrolled in, or a graduate of, a medical school accredited by the Committee on Accreditation of Canadian Medical Schools;
(c) A student enrolled in, or a graduate of, a school accredited by the American Osteopathic Association;
(d) A student enrolled in, or a graduate of, a school not accredited by the LCME, the Committee on Accreditation of Canadian Medical Schools, or the American Osteopathic Association; or
(e) A student enrolled in, or a graduate of, a Fifth Pathway program.
--By offering safety, some less competitive programs give prematch to attract candidates whom they may not normally get through normal Match. Because of this, some competitive programs also offer prematch to keep their highly-desired applicants from taking other's prematch offers.
--Not taking a prematch offer sometime is equal to telling programs that they are not your top choice, thus may jeopardize one's rank position if programs do care about that. I suppose if a program really like a candidate, they may still rank him/her high though. Also, sometime a good reason can be given to reject the prematch offer, such as in a couple's match situation, one may reject an offer simply because his significant other did not secure a position in the same city.
--For IMGs, rejecting a prematch offer may completely kill the chance of Matching at certain programs. This is because certain programs will quickly offer the position to someone else that may indeed take it. Some programs tend to fill a few positions by prematch and for the rest of positions, rank US seniors over independent candidates regardless of their credentials. I cancelled some of my interviews after learning that because I was certain about not taking their prematch offers, and I didn't think I stand much of a chance to match there. I was advised to go to places like that by some, because using their prematch offers, I might negotiate offers from better places. I was uncomfortable about this strategy and didn't take the advice.
--I finally took a prematch offer after going through all my interviews and was offered prematch at a program that I would clearly rank #1. I also know some others who took prematch at their top programs. In such a situation, I do consider prematch advantageous for the certainty it offers.