For example: in US layout, Shift-2 gives you @. But in Finnish layout, Shift-2 gives you ", and you have to press AltGr-2 to get @, because the letter ? is in place of the key that gives you ' and " in the US layout. Do you follow? I don't have to switch layouts at all, but I have to use AltGr more, because more keys have AltGr-accessed glyphs in them.
Indeed. There have been clamshell tablets where one half is a virtual keyboard controlled by a touchscreen, from what I remember. Though that's not quite as nice as using an actual keyboard.