The two things I've heard are that its for both keeping the noise down, apparently porsche decided that it was easier to have all their cars meet the strictest noise restrictions (I believe Sweden was the country in question?) and to keep foreign objects, such as cigarette butts, out of the intake.
Althought I have seen this on other forums:
http://www.theautobahn.com/forum/showthread.php?t=13930
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