I agree this feature would be very helpful. One workaround that I'm employing and may work in specific circumstances, subject to data and size limitations.
I used the "Raster to Polygon" tool to convert my rasters to feature classes in ArcMap (Desktop). Then I uploaded BOTH the raster and the new feature class as two separate services on ArcGIS Online. In my Online map, I left the raster visible, but symbolized the fill and outline of the feature class as hollow. Depending on the nature of the data, displaying the feature class probably would have bogged things down, but all I used the feature class for was to drive the value pop-up.
Worked like a charm; problem solved!