from a scientific point of view this is true because controls are impossible. from a practical standpoint most people are deficient so if the test convinces them to supplement, all the better.
whats the problem with the patch test? the Iodine can disappear for any number of reasons including high humidity and temperatures. where it does work is once you are Iodine sufficient(or thereabouts), that patch will last a long time, longer than usual under the same environmental conditions. therefor it does give a person a reliable gage of sorts.
so you see, from a lay person and do-it-yourselfer POV, it works fine. its just not accurate enough for doctors, and i cant blame them in this case.