You could not paste the segments together better. The geometry makes it impossible for all edges to meet perfectly. For them all to meet perfectly the two axis of rotation of the pancam would have to be exactly about the camera lens. It is almost impossible to do this in real life. On Spirit, the rotation points are slightly behind the lenses for azimuth. They are also inbound of each eye for sideways rotation, because it has 2 cameras for stereo vision (actually 4 including the nav cams).

You could past 2 pictures together well... but you would have lots of problems trying to paste 12 together. Actually its even more complicated then it appears because they are trying to make stereo images. Also, Each eye can take different parts of the light spectrum, you need data from both eyes to get the whole spectrum, and combing the color portions from different perspectives is interesting.