Actually it's very easy to open files in the trash. Normally when you double click a file, Finder examines the file to see which application to open it in, and then tells the application to open it. If it's in the Trash, it will give an error message.
However, if you bypass Finder, they can be opened very easily. For example, if you have TextEdit in the dock, you could drag a text document to it and it would open (note that dragging a file to an application's icon in Finder would not work, because Finder can still intervene). Alternatively, you could choose File->Open from an application, drag a file from the trash in Finder to the open panel (which selects the file in the open panel), and open it.
Even without a specific request to open a file, applications can very easily read files from the trash, and even write to them. So although Finder will try to prevent you from opening a file from the trash, very few other applications will, and you can't assume the files are inaccessible.