Created attachment 365217 [details]
Blue print of suggested dash behavior.

If multiple windows of the the same application are opened, left-clicking on an icon in the Dash takes you to the last-used window of that application. This isn't intuitive, but feels like the window shown to you is picked by chance. Especially if the windows are spread over several workspaces and many other windows are opened, the shell overview does not offer you much help to find the instance you are actually looking for.

To show the last used window might be more intuitive with a permanent visible dock, as in this case you would use the dock to switch quickly between open windows, but as Gnome users we are supposed to use the overview for this.

I think Gnome shell should rather show all windows of a certain application in the overview on clicking on their icon in the dash, than to raise the las used one. This would come in very handy, especially if all windows of the application from all workspaces would be shown in the overview. (By the way, way back with Compiz and Cairodock, this was the default behavior.)
To make it transparent in the overview that not all windows of one workspace, but all windows of a certain kind of all workspaces are shown, the workspaces which contains an instance of this certain application could be marked. To make clear, which application is shown, the icon in the dock should be also marked. To demonstrate how this could look like, see the following screen shot.
Say two instances of Nautilus are opened on two different workspaces. Clinking on the Nautilus icon in the dock would then open the dash showing both windows and marking the workspaces as origin of the window and the icon as the "filter" which is applied.