Streamlining and the Coronation Class locomotives
Stanier famously didn't approve of having the Coronation Class streamlined, and said that he considered it to be a publicity fad. Streamlining wasn't going to have an appreciable effect below eighty mph, and Stanier announced that while he would go along with the streamlining directive (because it was easier to placate fools than to reason with them), that he'd also build some "proper" non-streamlined locos (the first batch of which were 6240-44).
In a sense, Stanier was right ... the streamlining was about showing the public that the LMS could compete with their deadly rivals' streamlined A4 Class in terms of their trains' luxury, style, speed and modernity. However, part of that PR campaign involved being able to boast that your train was the fastest in the world, and although the Flying Scotsman had broken the 100mph mark without the benefit of any modern aerodynamics, every additional mile per hour made it more and more difficult to push away the wall of air in front of a high-speed locomotive, unless you made some concession to the shape of the front of your loco to help divert the air away and keep airflow under control.
Given that 6220 Coronation only just managed to squeak past the existing speed record by a paltry one mph (with the help of some slightly selective use of measurements), it's quite conceivable that if Stanier had had his way, a non-streamlined Coronation might have failed to accelerate fast enough to make use of the relatively short "good" bit of LMS track that made the speed record possible. Even though the track quality was the main limiting factor to the record attempt (requiring the loco to deliberately slow down before the end of its run before it hit sets of points and curves), if the train had been fractionally slower, LMS might have had to concede that their brand-new loco still didn't seem to be any faster than the existing competition, which would have been quite a blow.
Stanier vs streamlining
Stanier's opposition to streamlining might have been at least partly personal. As an engineer, he was proud of his loco design and wanted it to be seen, not cloaked under a fairing. As a locomotive engineer, streamlining would have been a comparatively new engineering discipline in which he might not have been particularly expert (compared to, say, aircraft designers), and expert engineers often have a reluctance to give credibility to new technologies that others have mastered better then them. From a designer's point of view, streamlining the Coronations was probably also a slightly frustrating exercise because the locomotive design was already so near the maximum size limit allowed on the British rail network that there wasn't a great deal of scope for creativity (apart from the shape of the nose).
Whatever his reasons might have been, Stanier seemed to want to have as little to do with the streamlining part of the project as possible, and was out of the country while the fairing design was being finished.
Whether due to Stanier's absence or some other reason, the engineering implementation of the Coronation Class' streamlining was perhaps not as complete as it could have been.
The curve of the nose had no obvious geometrical definition, and the builders in the loco works seem to have been given the basic outline and been left to work out the details themselves. While it was a tribute to their skill that they got all the streamliners looking "right", each loco nose was essentially a custom job, with no two exactly the same. The lack of definitions later became an issue for modelbuilders and probably deterred some companies from producing reproductions of the engine, since you couldn't simply build the shape from a blueprint without applying some additional artistry.
The first 1937 "blue" trains seemed to forget the principle that "proper" streamlining usually involves doing something to the back of the object as well as the front, and the loco design included the slightly perplexing engineering decision to fit the front of a high-speed locomotive with a pair of swing-opening front access doors, without a seam windbreak strip (which would have spoilt the smooth curve), or much in the way of reinforcing overlap. There are accounts of some locos having a nasty rattle as the wind got into the fairing gaps, and even of the front access doors occasionally opening at high speeds by themselves.
It's sometimes said that the decision to strip all the streamlining from the Coronations justifies Stanier's original attitude. The streamlining produced more problems due to access than it solved. However, this is perhaps more a criticism of the way that Stanier's design office implemented streamlining, since the LNER's competing A4 streamliners kept their air-smoothed fairings through the war and beyond, with the only serious modification being larger cutouts around the wheels. Perhaps it wasn't so much that streamlining wasn't "good engineering", as that some of the details of the streamlining produced under Stanier's unenthusiastic direction weren't quite as well thought-out as they could have been.
In the end, wartime and post-war speed restrictions (to 60mph or less) meant that the fairings now served little mechanical purpose. The smoothness of the design meant that there wasn't much to catch the eye once the silver and gold speed stripes were removed, and a Coronation loco painted in uniform wartime matt black was a fairly depressing sight, which also destroyed the "feel-good" aspect of the fairing. The Coronations' curved fairings were more difficult to customise for improved access, compared to the LNER A4s' flat wheel panels that could be almost completely cut away without destroying the look of the design.
In wartime and post-war austerity, where the trains that the Coronations were meant to pull no longer existed, where high-speed luxury rail travel was a thing of the past, and where the new priority was using the locos as workhorses with fast turnaround times and minimum servicing, it's not too surprising that when the railways had to choose between trying to redesign a fairing that no longer served its original purposes, or stripping it off and converting the locos to the existing (and perfectly decent) "Duchess" design, they eventually chose the second option.