There needs to be a way to post metadata files for download on AGOL hosted Open Data sites. (And as of ArcGIS 10.x, that means xml files.)
It is true that you get some metadata when you download a shapefile from the Open Data site. However, when I tried this on my site, all of the explanations for coded attribute values, which I had so laboriously typed in, were missing. Anyone else who downloads my data will not know, for example, that '1' means an elementary school, '2' means a middle school, etc.
The actual values of the coded attributes are established by the upstream scalar database that is the source of my spatial data, and I have to retain them. Thus, explanations of the attributes' meanings are vital to anyone using my data. The classic solution to this issue is detailed metadata.