My problem with Valedictorian being the measuring stick for scholarship is that what it signifies can vary so much, to be Valedictorian at a school that has 20 NMSF every year is a remarkable achievement, to be one at a small school with much lower ability students does not tell you much at all about the Valedictorian. It just seems random, at least NMF is some objective nationally standardized measure.
Also, just because the PSAT is done on one day, don’t discount the similar amount of hard work that went into having the ability to do well in it. I’m sure there’s a very high correlation between NMF and Valedictorians at schools that produce NMFs.