/386 contains much more than just binaries, while $home/bin/386 contains only that: binaries.
i think the rationale was that having a directory for each architecture back when 386 wasn't the dominant one would clutter $home. we rarely spend any time in / after all. besides, Plan 9 shouldn't expose the architecture to the user right at the front line: what the current arch is was not meant to be something we ought to care about. i could be wrong, of course.
