It is not useless to demonstrate that you do not accept a premise rather than (as assumed) being unable see the obvious logical consequences of said premise. It would lead them to disagree for slightly different reasons. If any part of such conversation is about sharing understanding and seeking to communicate information then Vladmir's comment is, in fact, rather useful.

(No, it will not convince anyone who wasn't already convinced. But that is because people are just not convinced about religion by argument ever.)