To suggest that one only needs to be able to see the detail that they require on the negative in order to obtain the photograph they want discounts the fact that the negative can retain more information than the paper is able to print. It's also necessary to match the detail (in the negative) that one requires on the final print to the paper that they've selected. I know that I've always had bad luck relying on paper grades to pull the proverbial rabbit out of the hat.

The zone system is an ingenious system to transfer the image onto a negative in a way that the photographer can best achieve his or her visualization of the final print. A densitometer is a useful tool that allows one to better and more easily accomplish this task.

As to Edward Weston, he used an emperical approach that relied on years of experience in the field taking how many thousands of shots in determining his exposure strategy. Using a light meter in conjunction with the zone system enables us to achieve a personal exposure strategy in far less time.