This is
an archive of a lot of Jobst Brandt's usenet debates. I don't see any place where he lays out directly why butted spokes (he refers to them as "swaged spokes") are better or worse, but he does address them in a couple of posts
here and
here. At various points, he seems to say that swaged spokes are better than straight-gauge spokes, but he also seems to be saying (and I hate to put words in his mouth) that spoke metallurgy has gotten so good that it doesn't matter.