While Sunn O))) definitely incorporates aspects of noise (perhaps a bit of performance art as well?), they nevertheless came from a distinctly metal background, and continue to incorporate metal into their studio albums. By consciously pushing metal music into the realm of the avant-garde, we have no choice but to recognize them as least one strain of the future of metal music.
I'm not sure what you mean by "useless." At least to me, the utility of a piece of music is determined by whether or not there is at least one person who is able to derive pleasure from listening to it. Since it is clearly the case that many people enjoy the music of Sunn O))), it seems to be improper to label them as useless.
Whether their riffs are good or not is subjective. I tend to like some of them.
There is still some controversy in metal today. Check out the Militant Black Metal movement.