Why do tonearm specifications list null points?

So, is the tonearm manufacturer trying to say that this particular tonearm is somehow optimized or was designed to utilize those null points they list in the spec?

Short answer is yes.

In theory you can run any alignment you want eg Stevenson, Baerwald or even create your own. The headshell has an offset, but due to the slots you can rotate the cartridge and move it back and forth.

However the pillar bearings for vertical motion are offset to match the headshell  and if you use another alignment that results in the cartridge offset being different to the pillar bearing offset then you introduce issues where if you adjust VTA then azimuth will change at the same time. Each time you change VTA you will need to recheck azimuth. This is not ideal.

Thanks for the explanation 

Dover, 

very good, and concise, explanation.  While the overhang for a different alignment scheme would result in a different angle of the cartridge that would not have the cantilever of the cartridge perpendicular to the axis of the vertical bearing of the arm, the difference in angle would be small, so that the imperfection would be more theoretical than practical.  There would be only a small variance in azimuth for different settings of VTA such that I would not worry much about it.