Maybe because in beta testing WRI is less interested in design suggestions and more interested in whether Mathematica crashes or performs incorrectly in some manner.
The style sheet changes were gratuitous and unnecessary - although I do rather like the Arial Text font. Users who design their own style sheets usually start with the Default style so when WRI changes that they are affecting a lot of users` custom work.
The problem with style sheets is that everyone has their own taste, which may be conditioned by what they are used to.
Nevertheless, I would like to suggest a few principles for good style sheets.
1) There should not be too great a variation in font size between a Title cell and a Text cell, else the text becomes too small or the title looks like a billboard. Differences in font style could compensate for smaller changes in font size. (WRI has a tendency to deemphasize text, either putting it in a small font or lightening the color.)
2) All fonts should be black. Color coding does not have an intrinsic meaning to readers. It can serve mainly to differentiate but is not needed on what is already differentiated. (Tufte's "minimum effective difference".)
3) All Section, Subsection, etc., headers should have group openers. Very intelligent readers may still not know about double clicking the cell brackets, which are not even normally visible. The group openers are more standard and intuitive. They are also an additional indication of a section in a notebook. But there should not be group openers on Titles, Subtitles and Subsubtitles or on Input/Output groups. (And I would prefer it if the Close All Groups menu command operated only on the various Section levels.)
4) Text cells should have the same font size as Output cells. This is the style in most printed technical material. Output cells should be unadorned and look like Text cells. Then, if the Input cells are closed, the document will read like a normal paper.
5) Text cell should allow a line-return without creating a new cell. Then it is possible to write extended semi-formatted textual discussion. Textual discussion is often an important element of notebooks, on a par with Input/Output. Default style behaves correctly here but some styles such as Journal Article don't.
The new stylesheet default.nb turned up in the last beta release. I was heavily shocked observing this. All my course material, using the old default style, now looks terrible. I wrote an angry reaction to wri, suggesting that at least users should have the choice using the old or the new default.nb, but never got a response.