Apparently, setting USE=x86 in make.conf on amd64 arch can have funny
consequences such as [1]. And I can imagine even more subtle and hard to
detect errors due to this.

I think it's better to prevent this rather than waste time with bug
reports like that. I asked Zac on IRC whether portage could filter such
flags. He suggested using use.mask in profiles. Well since ARCH is also
set by a profile, why not. Although a really persistent and stupid user
could use.unmask, it's better than no protection. And then we can think
how to replace the current ARCH->USE flag system with e.g. USE_EXPAND.
What do you think?

Vlastimil

[1] http://bugs.gentoo.org/show_bug.cgi?id=236801