What is better: place the using directives inside a namespace or outside?  

Most C# developers place their using directives outside the namespace, i.e. at the top of the file just before the namespace declaration, like so:

using System;
using System.IO;

namespace MyProject.Web
{
    public class MyClass
    {
        ...
    }
}

However, Microsoft itself seems to place their using directives sometimes inside the namespace, immediately after the namespace declaration. This which makes them part of the namespace I presume. Here is an example:

namespace MyProject.Web
{
    using System;
    using System.IO;

    public class MyClass
    {
        ...
    }
}

Why would they do that?
Are their any advantages to doing it either way?