Microsoft gave some explanation for these changes to Visual Studio in this article:
Visual Studio 11 User Interface Updates Coming in RC (May 2012)
(emphasis mine)
Another area of requested change relating to user interface
controls/chrome has been for us to improve the overall sense of Metro
styling within the themes by drawing our own window chrome. By drawing
our own window chrome we have succeeded in both making more efficient
use of space and in increasing the overall sense of Metro styling.
the article continues...
The custom chrome and line work changes we’ve made together with
reducing the number of default toolbars and toolbar icons combine to
give you three extra visible lines of code in the editor compared to
Visual Studio 10. As I noted at the beginning of the post the overall
objective behind many of the Visual Studio 11 theme changes is to give
you maximum real-estate for, and ability to focus on, your code.
It looks like Microsoft designed a custom title bar in Visual Studio 2012 to:
Move the "Quick Launch" search bar all the way up to the top of the window, so that users could hide all toolbars and see a few extra lines of code.
Make a more consistent metro theme.
Increase focus on "content" (i.e., code)
Based on the article it seems like the custom, low contrast title bars were Microsoft's attempt to "get you to focus more on your code" rather than looking at the window chrome.
Issues with Microsoft's article:
- If you hide all the toolbars, Visual Studio 2010 and 2013/2015 are very similar in the amount of vertical space you have for your code.
In the newer versions of visual studio, Microsoft made the tabs smaller, but they increased the size of the logo in the upper left, the notification button / indicator is also larger than a standard title bar button in Windows Classic.
For Classic Themes: Visual Studio 2013 and 2015 actually have one less pixel of vertical space than Visual Studio 2010.
(click on the image to enlarge it)

For Aero Themes: Visual Studio 2013 and 2015 give you four additional pixels of vertical space. (Default font, the pink line is the top of the i in #include ...
in Visual Studio 2010)
(click on the image to enlarge it)

This comparison was done on Windows 7.
In Windows 10 / Visual Studio 2017:
Compared to Visual Studio 2010 with Windows 7 Aero, you gain an additional 3 pixels of vertical space for your code.
Compared to Visual Studio 2010 Windows 7 Classic you lose 5 pixels of vertical space.
So far no environment has more maximum possible space for code as Windows 7 Classic / Visual Studio 2010, though the difference is negligible (not even enough for a full line of code).
As for "Metro makes it easier to focus on content":
A) In an article talking about Office 2007 (see below), Microsoft specifically mentioned that "Replacing the Window chrome is a very visible way to differentiate your app and increase branding impact"; it seems like if anything replacing the window chrome would make it so that users are noticing the unique look of your application more than the content.
With custom chrome, the application no longer blends in with the rest of the user's system (and it will probably only look more foreign later on, because it doesn't "evolve" when the OS updates).
B) Microsoft made the exact same claim about Windows Aero years ago (Archived: Oct. 2009):
One of Aero’s more visually obvious features is glass window borders, which let you focus on the contents of your open windows. Window behavior has also been redesigned, with subtle animations accompanying the minimizing, maximizing, and repositioning of windows to appear more smooth and effortless.
Office 2007
Joe Castro (Microsoft) wrote an article on the usage of custom window chrome (archived 2019), he mentioned Office 2007.
See excerpt below, he was mostly focusing on the technical side of it in the article, however, he mentions that replacing the window chrome can give your application a distinctive look, though it will require more work to implement standard system features. He specifically mentions Active/Inactive states as something requiring extra work.
It seems that some employees at Microsoft recognized the importance of differentiating between an active and inactive window, though perhaps a more "subtle" difference between active and inactive was preferred in the long run.
Predicting the future is hard – or “How to make some people unhappy,
all of the time”
Ultimately replacing the window chrome is doing the job of the window
manager. Emulating Windows like this has potential to miss behaviors
that your users expect, or not work correctly under some circumstances
(like high-DPI, or under a screen reader). Care should be taken to
ensure that the behaviors your users care about will work correctly
with your replacement.
For example, some of the standard window caption features today are:
Left-click on the icon to get the system menu.
Double click on the system icon to close the app (Office’s pearl does this as well).
Right click in the caption to get the system menu.
Double-click the caption to maximize the app.
With DWM, change the caption text style when maximized.
Change colors based on Active/Inactive states (The colors it uses respect DWM colorization when Aero glass is on, the Theme colors when
not in Windows Classic, and System colors when in Windows Classic.)
It respects system metrics for sizes, and the metrics available for measurement have changed in different versions of Windows (e.g.
iPaddedBorderWidth was added for Vista).
These are all things that the system no longer does automatically for
you when you use your own chrome. And some of this behavior has
changed in different versions of Windows.
Replacing the Window chrome is a very visible way to differentiate
your app and increase branding impact, but it’s likely that an
implementation will miss some things, or that future versions of
Windows may change some behaviors making your app look out of place.
This doesn’t mean “don’t do this.” Just that replacing the chrome
should be an informed decision.
Also, user feedback on Office 2013 has shown that low contrast in title bars is not just a hypothetical issue:
And another thing, now that I have to have all of these separate window open, how can I tell which one is selected? Previous Office products had a color change in the bar at the top of the window to show whether that window had the focus. Office 2013 doesn't seem to have any visible indication. I keep pasting things into the wrong windows.