Zeke, Lifetimedoc, Guju, I think you understand most of the good reasons for an SMP. Something I wonder about is the definition of postbac. When I use postbac, I mean a 2nd bachelors degree. With this definition the 2nd BS, usually a 50-60 hour degree, is good for those who either did not have a science degree or have been away from academics for awhile. I usually recommend a full "postbac" rather than a couple of full-time semesters of somewhat hap-hazard courses. In general med schools do not want you to retake courses unless you can justify it by fortifying a weak science foundation in preparation for the MCAT. Normally you should take higher level coursework rather than retake. If your undergrad degree was science and you didnt do a good job and there arent too many meaningful basic science courses left for you to take at the BS level, then a MS in basic sci is the best next step to show an admissions committee that you have the academic strength to survive med school academics. The other definition of postbac I have heard from some are some of the MS taken after your bachelors degree. I believe they should be called MS degrees, not postbac although they do occur after bac.
Now for those who might be the best candidates for an SMP which I define as a Med I curriculum, preferably with Med I's (some programs are purportedly the curriculum, but no Med I's). The most common situation that I would recommend an SMP to would be: 1) someone who has done a credible job in bs and probably MS basic science work (3.40-3.60 SGPA -- these would be state of Florida GPA's) and has taken the MCAT a few times and cant seem to get above 25-26 and just isnt getting the attention for interviews. At no time would I recommend the SMP to someone who applied only once and didnt attempt to repair weakness. If the are reapplying they probably havent displayed enough science ability in the GPA or MCAT. They must build a strong foundation before playing for end game or they are probably going to lose.