Many times users would like to use an attractive ESRI street map as a base map for their web app. It is, however, in WGS84. Applying ESRI template to your own data could be a painful experience, and adding ArcGIS Online street map to the MXD and caching on your local ArcGIS server is slow and distorts the original tiles, introducing noise.
Wouldn't it be nice to have an uncached ARcGIS online street map service? It'll result in a much better-looking reprojected base map (albeit just as slow to cache) as when using the original tile imagery. With a deal from Amazon virtual host program, ESRI may be able to provide all this extra processing required for cheap, or it could be a premium service.