passive noise-cancelling mics rely on those fins to operate. something about if the same sound exists on both sides of the mic (front where you speak, and at the fins) then it will get cancelled out, but sound directed at it from the user (front only) will get picked up.
if you take the package away, you just have a normal microphone which isn't always as good. if you're just holding it in open air, well then it has a vented rear already, but if you go mounting it in your car somewhere where the area behind the mic is sealed up, then it will probably pick up more noise.
But don't take it from me! here's a quote from a real, live newbie:
eegeek.netOriginally Posted by Viscouse