Creating an instance every time one wants to sort isn't that convenient (code usability for devs), especially when such class doesn't contain anything (except for cached instances).

It is more convenient to just be able to call:
usort($array, array('\OC\Util', 'naturalSortCompare')) than having to instantiate it first.

Should I move that method to the OC_NaturalSort class directly if Util is not the right place ?