Why is superglobal $GLOBALS['user'] recommended over global $user?

Drupl'Art suggests the following:

Consider $GLOBALS['user'] instead of global $user. [druplart_user]

The only explanation I have found [0]:

The global $user object is a source of security errors. Many a developer has accidentally overwritten or changed data on the global object, giving the current user more privileges than they deserve. Two simple patterns usually solve this problem: First, use $GLOBALS['user'] instead of $user. This simple change comes with a mindset that is read-only. Second, always name local variables that hold user data $account, never $user.

$GLOBALS['user'] is not read only, though. For instance, the following code modifies the user global:

// Modify global object: method 1.
$GLOBALS['user']->foo = 'bar';

// Modify global object: method 2.
$account = $GLOBALS['user'];
$account->foo = 'baz';

So, what is the value of this suggestion?

[0] http://drupalwatchdog.com/volume-4/issue-1/druplart