Trig,
I am with you on this. Pelz is not an optomotrist, (i think that is how you spell it) and neither am I; but, I do believe "see" benefit in it. The line of the putt is dictated by the speed of the putt which is dictated by the slope of the terrain and the "feel" for the execution of the golf stroke (even though it is a putt) itself.
I feel plumbing a putt is helpful at times. I have seen (read, heard) many different varieties and I believe it gives all of us a vertical line to find slope.
Geoff M. would be a better instructor to why it works for certain because of his research into eyes. I just know it has helped me get to the score's tent with fewer shots over the years.
Sustain our lag,
Trolio