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It's pripary purpose is IMHO to allow building of rpms when
you distribute some python stuff. It is good for this case.

The problem for distributions, is that all packages need to
be handled same way (in case they use some automatically
building system), so you can not differ whether it is
packaged some python stuff or kernel. So separate spec file
and sources is the only solution. Generating spec file
doesn't help much as it can get changed every version and
vendor has own additional information in spec file, which
would require hand merging.

Another idea I just got is to generate just file list using
separate bdist_rpm and use it separately, but I'm not sure
whether it is doable - information about file list is not
probably available there, is it?

It would be used like:

python setup.py install --root=$RPM_BUILD_ROOT
python setup.py bdist_rpm --filelist=INSTALLED_FILES