The issue Derick pointed out:
In PHP 5.3 this would alias the class Interval in the namespace PEAR::Date to the class Interval. For now, this code would work just fine. However, if PHP would introduce a class "Interval" at some point in the future (and PHP can do this as it owns the global namespace) then the above code would suddenly stop working.
Greg's patch makes it possible to "override" the internal class if need be, making it so that code currently working wouldn't break. All it takes is to "use" is like you normally would and and the magic happens behind the scenes.