It is a common knowledge ( and convention ) that namespaces are used to uniquely identify code and prevent name conflicts, so you should always put your own code in a unique namespace.
But ...
Namespaces are also used to group logically relevant classes. My personal library just extends to the class class library (FCL) and implements all the functions that I consider to be absent in FCL. It contains mainly extension and helper classes, as well as other highly reusable classes. I found it very useful to follow the same namespace structure as in FCL.
I went one step further and started using FCL namespaces to “enter” my code into namespaces, where I really expected them.
One of the advantages is that you do not need to determine as much as you like. For instance. simply referring to the library, extension methods are immediately available without adding additional messages. As a good example (for those who like to keep their users list clean), how often do you expect extension methods to work, just to make sure you don't add "using System.Linq;" yet?
UPDATE
So the main advantage that I consider (as mentioned in the comments) is that you have access to more utilities without having to look for specific namespaces. For instance. If you use System.Collections.Generic, extension methods are immediately available for IList<T>.
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using System.Windows;
using System.Windows.Controls;
using System.Windows.Media
using System.Windows.Media.Media3D;
using MyNamespace.Windows;
using MyNamespace.Windows.Controls;
using MyNamespace.Windows.Media;
using MyNamespace.Windows.Media.Media3D;
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