Crosstalk can cause nausea and headaches in some people, just like VAC and motion lag.
That being said, circular polarization off of a retro reflective screen is one of the two state of the art ways of doing step stereo 3d: if you saw Avatar in a modern theater, there's a good chance that was used, so crosstalk should be minimal and much better than with an anaglyph.
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I should also note that circular polarization is different from the linear polarization that was used e.g. in Disney attractions in the '90s (and thus still used in some of the older attractions). With linear polarization if you tilt your head 45 degrees to one side or the other you will get an even mix of the L/R channels in each eye. Circular polarization does not have that issue.
Though it seems to me with the projector originating from the same plane as the glasses, circular polarization would be redundant. I'm sure the engineers there know more than someone who dropped out of physics after flunking the optics lab though :)