i've ruminated about this, and though i'm pretty ignorant, my general rule of thumb is: the more tortuous the secondary, the more important it is. i don't think adcoms are so sadistic / masochistic that they'd make us write all these essays that they then have to buckle down and read, unless those essays were at least somewhat significant.
another thought: check out mdapplicants, and look at the folks who were rejected pre-interview but have pretty good stats and ECs - or vice versa (lackluster stats and ECs but got an interview). that might give some insight into whether the secondary was a big screening tool. of course, this is a very vague and probably inaccurate way to assess a secondary's importance, but it can sometimes while away the time on a boring sunday 😉
