The goal of this issue is to display the "Row Evolution" icon on all reports, when this report is flattened.
Currently we do not display the Row Evolution icon because the Flatten reports were too slow to generate. Therefore exposing this feature would have been bad user experience in many cases. Now that we have made Flatten much faster in https://github.com/piwik/piwik/issues/6763#issuecomment-77797219 then we could again re-display this icon. I believe that speed was the only reason we didn't display the icon (rather than having to write new code etc.).
The value for users is huge: for example seeing Evolution of metrics for Flattened page urls is priceless.