last time i checked, the alpha channels (g-buffers) weren't anti-aliased. so if you are using post render effects based on them (depth of field, light cone, lens flare, etc.) you will probably see aliasing in your final render that wouldn't be present if there were no post-render effects.

kix is right that carrara has long had poor anti-aliasing. I generally render my images 2 or 3 times larger than necessary and then scale down in photoshop. This will soften your g-buffers too.

-pix