Comment 11 for bug 633673

Yup, the main case for isa in BuildRequires is getting sensible behavior when doing rpmbuild for a secondary arch, eg:
[pmatilai@dhcp102 SPECS]$ uname -m
x86_64
[pmatilai@dhcp102 SPECS]$ rpmbuild -bb --target i686 rpm.spec
Building target platforms: i686
Building for target i686
error: Failed build dependencies:
 db4-devel(x86-32) is needed by rpm-4.8.0-2.fc12.i686
 elfutils-devel(x86-32) >= 0.112 is needed by rpm-4.8.0-2.fc12.i686
 elfutils-libelf-devel(x86-32) is needed by rpm-4.8.0-2.fc12.i686
 readline-devel(x86-32) is needed by rpm-4.8.0-2.fc12.i686
        ...

Without isa, you'd get varying errors in the middle of the build despite rpm seeming happy about buildrequires.

Of course whether %{_isa} is in src.rpm or not, this doesn't work with something like yum-builddep - as has been noted here the only meaningful place to figure buildrequires is at build, not src.rpm generation time. I wasn't joking about dropping distribution of source rpms and pointing people to dist-cvs instead.

Oh and adding an API to rpm-python for resolving build-requires from a spec wouldn't be hard at all, it's just not there yet.