I guess experienced users don't need or want the dialog, but new users may be expecting both projects to be open like many other IDEs and like `open`ing files. If the dialog is removed then when a project is already open the `Open Project` menu item should change to `Switch Project` to indicate to new users its not an additional project, oh wait changing menus violates G* rules ... by being useful and helpful </my rant>.
I like the suggestion by @kugel- of a "don't show this again" on the dialog, we are trying to not let stuff be heaped into `Various` prefs.