Your automotive analogy is misapplied. The differences between learning as a pro-am student as a member of an amateur couple actually resembles the differences between driving an automatic (the pro-am student) and driving a straight-shift.
On the contrary, yours is the "misapplied" analogy because it introduces another separate skill into the equation. The operating of a stick shift is an additional activity layered on top of the basic skill of driving. A professional teacher is not better than a student because he can layer an additional activity on top of his dancing, like say juggling bean bags. He's better because he excels at the same basic set of skills. So the most accurate comparison would be one where the parameters are most similar -- in the case of driving, one where the basic function of the cars is the same, i.e. both automatic.
This brings us back to the simple premise that if you are reasonably well taught, making the transition from pro to amateur partner should not result in the complete inability to dance, as you suggested.
We can also look at it conversely: If you've witnessed the complete breakdown of a student when put in this situation, what you've seen is someone who was not, in fact, reasonably well taught.
However, if that's the case, then it was the ability of the teacher (or perhaps even the student) that was to blame... not the fact that the prior training was pro-am in nature. Chances are that if that same student had switched from one amateur partner to another, under the guidance of the same unskilled teacher, you would have witnessed the same breakdown... possibly worse.
Students learning with other amateurs learn their mechanics better because they must do so. Teachers function most effectively when they can observe their students without having also to serve as partners.
All arguments in favor of learning as a pair can be countered either with arguments against it, or arguments in favor of learning as a single. I'll take take your points above as examples:
#1 Students learning together learn the mechanics better.
FALSE. Concepts are absorbed much more effectively when undisturbed by outside noise, such as a partner whose own problems aggravate your own. When the second half of the partnership is relatively problem-free, the learning process is quicker and more concentrated.
True, having someone compensate for your weaknesses will not enable you to learn. But that again points to the deficiencies of the teacher, not to those inherent in pro-am dancing. A good pro-am teacher will recognize problems and fix them, not by compensating but by teaching. And bad teachers are bad teachers, whether they work with singles or couples.
Also not to be overlooked: By sharing the lesson with someone else, you get exactly half the attention.
#2 Teachers function more effectively when they can observe their students.
PARTLY TRUE: Some aspects of dancing are more easily observed from afar; Others from close-up, and others still can only be felt by the partner. Couples' teaching methods tend to be better with the visual, while singles' teaching methods tend to be better for the partnership and feel-based skills.
You also have to realize that the each method can be used by either party. It is as simple a task for pro-am teachers to use various methods to enable them to get a third-party perspective as it is for a couples' teacher to insert himself into the partnership to get a feel for what's going on inside. The best teachers employ all methods, selecting the ones that best suit each situation.
For outside perspective, pro-am teachers can observe a student dancing solo, watch a video (either of a recent performance, or one taken on the fly in studio), and of course, hire a third party for a coaching lesson. Coaching is generally much more effective with a pro-am teacher as a partner because he's in the best position to retain and later reproduce the information learned on the coaching lesson, being that the bulk of the information is not new to him; It serves as more of a guide.
Finally, and most importantly, is the skill of the pro himself: Having already put in the time required to learn the skills, he doesn't typically need to physically stand outside the partnership to know exactly what it looks like. The more skilled we become at our craft, the smaller the gap between what we imagine it looks like, and what it actually does look like.
You can argue all day long about the merits of learning as a couple, and it's easy to counter with arguments in favor of learning alone. But that will quickly grow tiresome. I think it's much easier if we all agree, as I said before, that there are relative strengths and weaknesses to either method of learning. In a perfect scenario, one would do both and capitalize on the benefits of each.
(Continued in next message...)