what to do with what stats is up to the developers the idea is to make more stats more useful. It was just an illustrative example, and even in that case, half your vit gone will effect your max hp, your total defense, in exchange for a few more seconds on sentinel?
also the idea is usally that in order to raise one stat really high, it cost way more than raising a lower stat. So while yeah vit may be better than mind in general, is 1 vit better than +6 mind? There would also be a difference between an overall tank, and a tank that tries to tank mostly from buffs. the buff tank would probably have lower overall defense, and lower hp, but it would have longer durations on skills, giving them a few more seconds and making thier defense be about rotating and stacking skills. So there would be two different styles of tanking for different types of tanks.
Sure people will always create cookie cutters and try to use other peoples builds, but that doesnt mean there are no other viable builds, or builds that would have a slight advantage.
The main problem is they would have to make it harder to respec, because if you can respec instantly at all times, people will just tailor make themselves for every encounter so if you wanted to go on a tour doing ifrit garuda and moogle battles, you couldnt just tailor make yourself to that one monsters weakness.
having a more rare or difficult respec would cause being all around to have a certain value, and being specific to have a different type of value.
Really your main beef seems to be that you want to min max easily. where only a few variables effect what you can do, if your really a skilled min max person, you want a complex formula that you can really build the perfect balance by combining everything just right. rather than just pumping strength and calling it a day on DD , or vit on a tank.