....The line is a management training program... In most US jurisdictions it even includes formal management training courses by the GL, though not usually called that.
I think that is spot on Dfreybur (we have Warden's courses as well).
Almost every US lodge I know of at least wants a progressive line. The only exceptions I can think of are Traditional Observance or European Concept lodges,. The one TO lodge in Illinois I have many friends in has the written goal to have the best qualified Brother in each chair but in practice progresses their line.
I thought that was an interesting statement - about having the best officers. American EC Lodges are often interested in what we do, because we have long run our lodges on those lines by custom. ( A dinning lodge here is really exactly an EC Lodge, and Victorian Lodges such as Lodge Epicurean, Amalthea, and Lodge Golden Fleece were often written about when the EC & TO Lodges were being discussed years ago.. A Brother like Kent Henderson founded Epicurean in Geelong (a regional city close to Melbourne) was often quoted in American literature on EC Lodges; he'd been a member of the Fleece... Golden Fleece #300 is an old lodge (1922) and I understand it has always had its orchestra and fine dining... What I found interesting was the statement "the written goal to have the best qualified Brother".
Our very widely held view is that the line ( IG, JD, SD, JW, SW, to WM) equips a brother to be in the East as WM. Each serves as a building block, much like moving from minor to major parts in a play. For instance, a Junior Warden controls an Inner Guard - that is much easier and clearer if, as JW you've been IG because you (should) understand his job and be able to direct him when taking reports and admitting visitors. What was interesting was "the best qualified". As a training system, it is expected the brother occupying an office will need to be trained and also will certainly need independent preparation by him using the ritual book and also our book of workings (the latter has more detail on moment). Attendance at rehearsals and meetings and effort are key, perhaps more so than talent or even success. If you are turning up and trying and showing some improvement, its unlikely you will be removed from the line. The move from SD to JW is key, as in taking the warden's position, the lodge will have a clear idea of if you aspire to the chair, but even a IG is generally asked about that before appointment, a more junior bro (one initiated later than you) will be selected for IG of he says he would like to be Master and you say you do not - it's not just a training, but also succession line.
There is also a clear expectation that MM should occupy line positions above PMs; as a line to develop masonic knowledge, it is assumed a PM already has that knowledge and hence does not personally need to be in the line. If PMs are in the line, it should be because there is no capable MMs available, and in "capable" I mean able to attend and willing to be trained with some degree of ability to retain the benefits of that training. Installing a mediocre MM is seen as more preferable than installing a very talented PM. Lodges that keep installing PMs simply die, because their MMs loose interest and often move to a lodge where they will "get a go" at things, including the East. Here, it is always noted favorably if all progressive (line) officers are occupied by MMs. In my two lodges, we only have a single PM in the line (he's SW in one, very keen, come from UGLE so is re-learning the ritual which is close, but not the same - probably making that job even harder! He is not PM of our Jurisdiction but retains the rank; and he's keen to give everything (and body!) a go at things).
I guess that might help the OP in explaining one set of thinking. Certainly, WMs should not play favourites and act as per Solomon's traditional reputation - with dispassion and wisdom, although that can be hard and in forming a team, you are also likely to look at its working as a whole, appointing two line officers who do not get along can be very problematic (its rare bros do not practice brotherly love but I have seen it in a line).
It might seem a big deal as a MM, but a re-arrangement of the line which see you out of it should only be seen as temporary, and unless there is some significant fault in the man, he should ask for charges and other jobs, regularly attend and support the lodge, and he should find himself back in the team of a good lodge.