The main point of invoking this alleged new user --however obliquely -- seems to be to provide another level of rationalization for design decisions, especially ones that remove features that more advanced users prefer, such as customizable panels with applets. Perhaps the simplifications associated with the new user might eventually attract actual first-timers, but the main role of the new user right now appears to be to justify the removal of features that existing users want.
That brings up yet another point: although impartially a feature might be a better design, it might not be so in historical context. For example, placing the main menu in the upper left corner of the desktop may be sensible for English speakers because that is where the eye first falls when reading, but is difficult to do because years of using Windows has conditioned millions to looking for the menu in the lower left. At the very least, you need to allow readers the ability to move the menu where they prefer.
Yet this context is almost totally ignored by both Unity and GNOME 3. The simplification of the panel, the repositioning of the task bar, Unity's title bar buttons on the left, GNOME 3's overview -- all these innovations and more not only change navigation on the desktop, but do so while not allowing any alternatives.
And from the perspective of new believers in usability, why should alternatives be allowed? After all, the innovations must be superior to any alternative, because they are supported by usability data. Never mind if the user prefers another configuration -- as newly taught usability experts, the developers must know what is best for users.
An over-emphasis and over-application of usability principles is not the only reason that users object to new desktops. A large reason is probably a reluctance to change. Possibly, too, what might seem like a user revolt can be just as easily explained by the fact that the free/Linux desktop is growing, and attracting a greater diversity of opinions. In other cases, such as KDE's, inflated expectations explain the initially hostile reactions.
At the same time, mainstream usability studies are in many ways foreign to the tradition of Linux. They are generally based on operating systems where restriction of choice is a norm, and the findings of many studies may not be wholly suitable to platforms with a greater tradition of choice.
In this respect, I cannot help but noticing that KDE remains the major desktop least affected by the arrogance of new experts. Admittedly, some KDE developers do have design expertise, but, so far as I can see, that expertise does not generally form the justification for design directions. Instead, KDE still seems inclined to add features because individual developers would like to use them, and to allow alternatives.
In fact, if anything, KDE's design principles -- including the creation of hardware and software sub-systems -- seem designed to encourage alternatives. The overall effect is not as streamlined as GNOME 3 or Unity, but, as KDE has matured from its 4.0 release, the result has been a desktop with more diversity than the current editions of GNOME or Unity.
No one would advocate a situation in which menu structures were different in each application, or the keystrokes for copying and pasting were different. All the same, it sometimes seems that uncritical acceptance of usability principles in the free desktop might be less rationale than is usually claimed -- and, if not carefully watched, as much an obstacle to the development of the free desktop as a help.
Possibly -- just possibly -- mainstream usability may have less to teach free desktop developers than many people assume.