In this case the polygon is so large that it only appears on lesser zoom levels. Giving coordinates would direct mappers to a location far too precise to illustrate the large scale of the erroneous polygon.

I agree, however, that geotagging is good practice for diary posts. I'm afraid I am limited to Potlatch at the moment (I'm on Mac OSX.3.9) so I'm not particularly familiar with the differing needs and capabilities of JOSM users.