OK, I can't resist... I'm gonna jump into this just one more time... This topic has had so many reincarnations, you'd think it would have reached its Omega Point by now!
Is there a time and a place to use an extension movement to facilitate a transition? You bet! Is there a time to use flexing movements? Again, the answer must be YES! To not be able to select which of these options (among others) might be the most effective or efficient in any given turn, woud be to grossly short change yourself from becoming a more versatile skier.
IMO, it comes down to what your needs are during the transition. Are you wishing to create energy? Or are you trying to manage the existing energy? This must be the initial criteria.
Can we stipulate to the fact that a good turn requires a certain amount of energy for everything to work as effectively as possible? If so, then we must be able to recognize whether the amount of energy existing in the preceeding turn puts on us one side or the other of the dividing line which determines which movement pattern will be most beneficial.
This energy can be found in multiple forms. Any one (or combination) of the following can contribute to the generation of energy.
-Radius of turn (shorter generates a more noticeable build up)
-Terrain (any undulation can create additional energy)
-Snow condition (harder snow can create energy, while softer snow dissipates energy)
If the amount of energy generated by the aforementioned factors is at a level where managing that energy is of concern, then a flexing type of movement may be in order. But in the absence of enough energy to facilitate the transition, then the skier must create additional amounts by using muscular effort.
So, imagine making slow, LR turns on a smooth, gentle slope covered with soft snow. This is one extreme where an extension movement would be of benefit by creating some of the energy which might facilitate an easier transition. Now imagine skiing quickly, making SR turns on a steep, bumpy run with hard snow. I think that you can imagine how much energy exists in this run! So managing that energy at a controllable level is the need. And by all means, I agree that a flexing type of movement is the most effective in achieving that control.
Now back to the real world.....
Depending on the various factors generating the energy in the turn, usually sufficient energy exists to justify the use of a flexing movement pattern. But harnessing and controlling this energy at a useable level requires a degree of practiced timing. For too quickly/too slowly or too early/too late, and its blown. At this point your options are somewhat limited. And the easiest way to get back on track is to use an extension movement. But before you remind me that it was a WC skier who is demonstrating the use of an extension movement, I will suggest that his idea of a "sufficient amount" of energy and yours are light years apart. So where the average skier skier might have more than sufficient energy pre-existing, the WC'er may desire more. And there are not many ways to get it.
But for those skiers not forcing themselves down the hill, who are happy to be passively pulled by gravity, the extension type of movement is a perfectly valid and acceptable method. It is easy to learn, requires minimal effort, has reasonable results as far as facilitating a connection from one turn to the next, and does not require a superman physique! So let them use it! It's their vacation!
This latter group probably encompasses 80% of the skiers on the hill. Maybe you believe that number to be to high? From decades of watching skiers and teaching all levels of students, I think that percentage to be a reasonable guess. (if anything, it is low)
The average EpicSki skier is not the average skier nationwide. Here we find those who want more out of their skiing, and are willing to spend time researching and writing about skiing. The average Joe Public skier does not do that. Depending upon whose figures you use, there are between 8 -12 MILLION people in the US who claim to be skiers. If members of EpicSki were average, we'd have MILLIONS more posting on here. So this small group of almost 17,000 can not be deemed "average".