weems, I disagree. The picture is designed to help people create a mental model of what is going on in the turn. And because it is a picture of the Herminator it is not just creating a mental model of what is going on, but also implying how one should apply forces in order to ski like one of The Great Ones.
The first thing to remember about models is that, as you and michaela note, they are by definition simplifications. As a rule when creating a model you throw out all the irrelevant distracting stuff that adds complexity but does not communicate anything important about the phenomenon you are discussing. And then, to the extent appropriate, you simplify what is left to the greatest extent possible to create correct understanding and communication.
In the case of the picture being discussed, the first obvious simplification is the projection of a 3D event that flows through time onto an instantaneous snapshot in a 2D plane. No problem - it is, at least to me, a natural simplification. In addition, hundreds or thousands, of vectors are reduced to 3. Another natural, and, to me, understandable simplification. At least in principle.
Where it falls down, IMO, is that the mental model created by this specific picture/diagram is one of the skier directing all force through the outside ski. In effect sending the message that "monopodal" skiing is how it is done. But this is a fundamentally flawed model. As far as I can tell, all of today's best skiers use a primarily two-footed stance. Now Rick noted that in this case most of the force appears to be applied through the outside leg - but what the skis are doing says that at least a bit is applied to the inside leg. Even if most of the force is driven through the outside ski, it is a fundamentally different model than "one footed" skiing. So... I'd argue that a better simplification for modeling purposes would have been to drop the vector dead middle of the skis. Even if it would have been further off or literally "less correct" in terms of force distribution, it would have caused people to think about a two-footed stance in the context of most of the turn.
And I think this matters in practical terms. Not all that long ago when I started skiing, I was led to believe that carving was all about lightening up the inside ski & relying on the "inside" edge of the outside ski for all important aspects of engaging the snow in a turn. Just as that diagram and others like it would make you believe. Long story short - once someone started to teach me about skiing with a two-footed stance my skiing improved a great deal. But it took me a full two seasons to rid myself, mostly, of old habits. What a waste or time and life and energy. Furthermore, I think this has practical implications with respect to skiing on modern equipment. It is my opinion, after reading many pages of threads discussing recent injuries suffered by veteran skiers, that one significant contributor to these injuries is the use of modern highly sidecut equipment with relatively old-school technique. That leaving the inside ski to its own devices increases the likelihood of unexpected hookup & an assortment of ensuing possibilities...
Soooo.... I agree that simplification can aid understanding. I just think that simplifying in the wrong way can create or reinforce incorrect "understanding". As this diagram does.