IMHO to really follow the hamcrest matchers design philosophy the argThat interface should look like this: public static <T> T argThat(Matcher<? super T> matcher), but unfortunately it is

this change would allow a statement like the following:
verify(mock).aMethod(argThat(nullValue()))

I see that the default value is derived from the concrete matcher type, which of course would not work anymore. To circumvent this problem, it would be possible to provide a overload for primitives types.

Or provide a new argThat method, for example argThatSuper which always returns null, but of course would not work for primitive types. Than the existing argThat would keep its behavior.