Some third-party repos (e.g., livna) don't have repos for all available "official" archs (i386 and x86_64 for the moment) yet are included in commonly-available yum config changes/addons. It would be convenient to have an "excludearch" or "exclusivearch" tag that could be used in these configs to indicate that certain repos can only be used for certain archs.
yum only uses archs that are compatible with the platforms its running on - so if you have a repo with exclusively ppc packages and you're an p4 - it won't use any of them. What's the problem that this feature would preclude?
While yum won't allow you to install a ppc package on an i386 platform, if the repo path contains $basearch it will still attempt to descend into the i386 dir, even if only a ppc dir exists. It will then of course fail to find the repodata, and abort the entire operation outright. While it could be said that your yum config shouldn't contain repos that don't support your arch, it would also be convenient for .repo distrobution purposes to have some way to desensitive yum to this sort of problem.
if your repo is arch specific then just distribute your .repo file in an arch-specific rpm. I think adding arch exclusivity in the yum.conf is overkill.