I agree that it's unnecessary in terms of where he will eventually wind up, but that doesn't mean that there will be no benefit in the meantime. He'll attain full proficiency faster, and in the meantime his speed and accuracy will be improved vs. with no instruction. The lessons go quickly. I don't see an important drawback.
With a struggling speller I would take a different approach, probably doing it more consistently but over a much longer period. With DS6 we just blow through it quickly.
ETA: I also didn't find spelling instruction a good use of my time in school. I remember kids at my school being taught by what I believe was called the "Skyline" method at the time, basically memorizing words by the heights of the letters. It seemed silly to me and still does. What I do with DS is different: I give him a word, he quickly spells it, and if he gets it wrong (which is pretty rarely) I ask him to try again. It has made him a lot faster and has filled in chinks in his armor, so to speak.
The increase in accuracy helps because he's loath to do things where he might be less than perfect; for writing I decided that precision was a good thing to be fostered, and he has the capacity to actually be perfect with grammar and spelling, so we might as well go for it. It may have a secondary effect of impressing the teachers more at his school, but the reason we began was just to help him become faster and more self-assured while writing in the classroom.
