Or, consider the trickster, the seventh--for me, Fe. Using this process I can fool myself into thinking something is important when it isn't. Like, say, by imagining that for the sake of efficiency and effectiveness as far as other people are concerned, it's important to be polite--or perhaps if something is important enough I can be polite and concerned about harmony? Nope, actually, that's too thoughtful. More like, someone looks at me and they have some concern or query in their eye and I get upset because I know I can't make them at ease so I try to be smiling and happy anyway for the sake of just not getting upset anymore even though, actually in practice, it never works well but I haven't got any other resources...
That last one in particular is either wrong as a presentation of when a shadow function is working for me or it's proof that Beebe shadows aren't that impressive because that "feeling" described above is easily conceived of as some combination of Ni/Te/Fi/Se all working together normally (or as normally as some combination of mature/growing/immature/basementdweller functions can.
On the other hand... if to have Xy function really means that you have X function just like anyone else with X function except you happen to have a heavy y focus just so that you actually have a functioning function (because without direction, consciousness doesn't, um, "conch", a made up verb to instantiate a claim that consciousness that is static isn't consciousness), then technically, given some consciousness shaking circumstances, you can access the opposite focus too if you like.
Which is to say, it needed be the case that the top four "functions" explain everything. As much as one would like them to, or as much as introspection seems
to point that way, shadows aren't ruled out. Indeed, introspection could sort of be expected to hide the shadows. Introspection would be keyed to the familiar functions (or function orientations) as explanatory.
How about "Use it or lose it"? The function orientations that oppose the function orientations of your top four "functions" are rarely (if ever) consciously animated. Blink and you miss them. (Or blink and you disguise them as what they aren't, your normal functions.) So how do they contribute? To call the unconscious mind "archaic" seems a bit of an understatement.