Some schools will send secondaries to every applicant, while others will pre-screen and send secondaries out only to those applicants they would potentially consider for an interview. Try to find out which secondaries are which. Then look at the stats of last year's incoming class for all the schools which sent you a secondary. If you can afford the money, more or less, send out all the secondaries you can, even the ones that seem kind of remote. The worse one's stats, the more chances that person needs to take.
Secondaries are a good money making thing for schools which do not pre-screen. Yet, the admission game is also somewhat random, so it may pay off to "gamble".You may happen to fall into a "desireable" category for a particular year. That means, most schools try to aim at a diverse class, and perhaps this year they are trying to have more students from a particular area of the country or with a particular background, and you happen to fit the bill.
By the same token, one has to be somewhat realistic as to his own chances for admission. I haven't met anyone yet who has been accepted with a GPA of 2.5 in recent years. I have met a few students who were accepted at different schools with an MCAT around 23 or so, and one with a 21, but their other qualifications more than made up for the MCAT score. The person with the lowest science GPA I know, who has been accepted to med school, had a science GPA of just barely 3.0, but had some very extenuating circumstances attached to it.
