PHP: Using only static method classes?

Question

What are the drawbacks to using a class with only static methods in PHP as a wrapper for common functions? Especially if I don't need more than one of something, or I want some sort of global controller.

For example:

class SomeClass{
    private static $somedata = null;

    public static function setdata($value)
    {
        self::$somedata = $value;
    }

    public static function getdata()
    {
        return self::$somedata;
    }
}

class AnotherClass{
    public static function modifydata($value)
    {
        someClass::setdata($value);
    }
}

SomeClass::setdata('firstval');

// returns 'firstval'
echo SomeClass::getdata();

AnotherClass::modifydata('secondval');

// returns 'secondval'
echo SomeClass::getdata();

I don't see this done commonly, and it seems like a pretty simple way to handle things, or am I just completely off base? What are the advantages / disadvantages to approaching things this way when building PHP applications?