I think it is a great idea. It is an omission that should have been corrected long ago. With respect to results vocabulary we are still in the MDG era. Not really acceptable is it?
Your suggestion to refrain from adding the SDG results vocabulary falls short in many ways. A sector vocabulary is not a result, it is an input, an intention. So you could use the sector vocabulary to show how much you are investing in the various SDGs. But this does not say anything on the results actually achieved. And this what we want to report on, using the indicators as agreed upon in the SDG framework.
In addition, I hardly know any publisher that is going to sector code its activities twice; once against the OECD crs purpose codes and once against the SDG sectors (??). This why some are suggesting a mapping of the crs purpose codes against the SDGs.
In short, having it as a sector vocabulary is nice to have, having it as a results vocabulary is need to have.