Some AF/AE setups in cameras use polarizing beam splittter and there is better to smear out the linear polarization and make it more or less circular.

Quoting Wiki here:

"The additional step of converting the light to circular polarization avoids problems with auto-focus and light-metering sensors within some cameras, which otherwise may not function reliably with a simple linear polariser."

http://en.wikipedia.org/wiki/Polarizing_filter_(photography)

so it seems that many light detectors do not work correctly with polarized light, so they polarize it circularly with a 1/4 wave plate after the linear polarizer (which is used to increase contrast, reduce reflections or reduce intensity if you use a second one as far as I know)

If we're quoting wiki:

"Linearly-polarized light may also defeat the action of the Anti-aliasing filter (Low-pass filter) on the imaging sensor."

AA filter is made of thin birefringent layers that spreads out point image on four spots spaced less than pixel size. If CCD physically big and its resolution is small, it may be a problem.