All I have to say is my horse had cracked hooves like no other. His past owner put him on Pure Methionine powder and vowed to shoot me if I took him off of it because apparently his feet would go bad.
I had him 2 months and with good 4-5 week trims and a better diet, he hasn't cracked since. And he's off of that stuff.
I think it boils down to diet and partly genetics. Hoof supplements are supplementing.. things that are lacking. That was worded very awkwardly.. but think about when you were little and had those wooden shapes that would fit into holes. Yes you could fit the triangle in the square hole, but there were gaps. Same thing.. An example: If we don't soak up enough vitamin d, there is a supplement we can take (a vitamin pill) to help with the lack of it. Yeah it's easier to go out and stand in the sun for 10 minutes, but some rather take the vitamins maybe due to their lifestyle or maybe they have a condition where they can't be in the sun or perhaps they burn easily.
Same thing with a horse, if he's lacking SOMETHING in his diet, which is apparent by the condition of his feet and perhaps his coat and eyes too, then you could supplement.. but I'd focus on trying to fix the gap in his nutrition.
Honestly, Beauseant, I asked about my horse's hooves being different because nothing is 100% equal on each side or each hoof. But I know you trim each hoof not based on a cookie cutter shape. You can modify the shape to alter the balance.. which may alter the way the hoof wears, but they all shouldn't be EXACTLY the same. It should be trimmed according to the horse.
And I'm not an expert farrier or whatnot, but I do love to learn and ask a ton of questions (my poor farrier :P) so I can know how things work and why he's doing X to my horse.