The Unity plugin is installed in far, far fewer places than Flash. That's why so many Unity devs have been asking for Flash export from Unity.
This 9% cost will affect all those developers. They are not a small group - Unity would not have put the effort it did into supporting a new (and very different) platform, Flash, if it wasn't for a large group.
You might want to export to Flash if you're going the licensing/sponsorship route ala Armor Games, Kongregate etc but there's not a ton of demand for the 3d stuff anyway.