The publishing 'on premise' of KML Map Services via ArcGIS Server is fully supported. This is great as it recognises that 'non-gis' users of spatial data may prefer to use Google Earth as their default client but still access SDE stored 'source of truth data'. The ArcGIS Server KML enabled Map Services support Google Earth users connected to the internal business IT network
ESRI needs to better support Google Earth for disconnected staff who are external to the network via ArcGIS Online. This can be done by enabling KML Feature Services published in ArcGIS Online.
Although KML can be uploaded as individual files the following limitations severely restrict usage: 1) There is no arcpy method to automate the update the hosted KML file when data source is updated 'on premise'. This means it needs to be manually re-uploaded at least once per day to keep data synchronised. 2) The single Feature Service on ArcGIS Online should support multiple clients. Currently multiple files/services need to be uploaded to ArcGIS Online to support different client applications. This adds an unwanted administration overhead 3) The size of the KML file that can be uploaded is restricted to a certain size. This means that as well as having to manually upload new files everytime source data changes, the process involves splitting source data into multiple KML files. There is no restriction on size for non-KML Feature Services published in ArcGIS Online.
For the reasons above please enable KML Feature Services published in ArcGIS Online ASAP