What you've said is of course basically correct. That at only two points (at best) can a Stylus be correctly aligned to the groove with a Pivotal Tonearm.
I'd probably be correct as well, saying that a large number of us end users, that were using poorer quality tools for alignment, never even succeeded to achieve correct alignment at least at those two points. For those that actually did, when they eventually compared to a better, more accurate tool (such as the Mintlp Tractor) dare I use the word "luck"?
The question arises, then just how accurate must one be? How much error will then impart a noticeable degradation of sonics? Some say accurate alignment should be within +-.5mm. But, perhaps what if the tool istelf has its own inherit error of poor resolution, and not designed-manufactured well enough for repeatable accurate results?
Of course again correct, an end user must conclude-decide what is the worth-value of a highly accurate alignment with a well made tool. Or take the other path-philosophy that says "My Technics Plastic Jig, or a lesser protractor is good enough, and the rest must be poppycock"?
All I can say from personal experience, using a lesser, but thought to be a decent tool, the Turntable Basics Mirror Sight Line Tractor, I used this tool a number of times, and every time I used this tool, I came away with what I saw as minute error, and a uncertain interpretation of that I had precisely aligned my Cartridge-Stylus. No doubt an inherit flaw of such tools, is the repeatable accuracy of aligning the site line, to center of Tonearm Pivot.
All one is doing with such a tool, is "guessing" where the center of Arm Pivot is. An ever so small error in aiming of course equated to an error of where the Null Grids would be positioned. Not only skewing correct overhang, but zenith angle as well.
After the use of the Mintlp Tractor, for the very first time, I did not feel compelled to re-check, or have a doubt that I did finally succeed in correctly aligning my Cartridge. Versus the TB Protractor, the Mintlp Tractor did indeed show the errors, in both overhang, and zenith angle.
All here who have used the Mintlp, including myself have noted worthy sonic gains. The value of those sonic gains are again up to an end user to determine what thier value is? Mark