It seems this problem is not related to the use keyword but imported 
items in general:

file: test2.php
------------------------------------
<?php
namespace Foo;

class Bar { }


file: test.php
------------------------------------
<?php
namespace NS2;

include 'test1.php';

use Foo\Bar;

$class = "Bar";
new $class;


Results is the same:
---------------------------
Fatal error: Class 'Bar' not found in /tmp/test.php on line 9

Call Stack:
    0.0003     632520   1. {main}() /tmp/test.php:0

Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

This isn't a bug, see Example 3 @ http://www.php.net/manual/en/language.namespaces.importing.php and the note "Importing is performed at compile-time, and so does not affect dynamic class, function or constant names."

So your code is referencing Foo in the root namespace, which is the documented behavior.