Yes, I agree that they are not exact for any elements. However there is a wide degree of difference in accuracy in say, calcium or magnesium, compared to the accuracy of the test for iodine.
And, the problem is exacerbated by the fact that a little too much iodine and you can poison your tank, whereas a little too much calcium or magnesium might cause some precipitation but it's not a potential doomsday like the iodine MIGHT be.
I guess it boils down to first of all, if one believes dosing iodine to be necessary, and then if so, does one trust the test kit for that purpose.

I dose iodine (and iron) without testing, I have for years. I dose the recommended amount on the bottle with my Mg supplement, it makes the SPS colors brighter in my tank. If I noticed any detrimental effects I would stop... Observation is your best tool in reef keeping.

[QUOTE Observation is your best tool in reef keeping.[/quote]

I dosed iodine for years as well with no adverse effects. I no longer dose as I cant find it anymore. Other than once a month I only test when I see something wrong or out of place in my tank. This hobby/obsession is a little too time consuming to begin with.

Im dosing with it -mostly LPS, I read the lighter hard corals can go blue if overdosed - i have noticed my tan/pink Monti now has some blue coloring spots so Im going to back off and see if it goes away - Ive been doing Lugos dosage as per directions