Certainly Denethor would have been much more helpful in the War of the Ring if his mind were not muddled like it was. The Palatir that he was using not only let him get eye to eye with Sauron (haha, that was bad) but also made him weaker in the mind. This led to him making faulty decisions. Who knows what he would have been able to do to help against the problem with Mordor if he hadn't been chatting with the Dark Lord in his spare time.
I'm not sure if he would have actually realised that destroying the Enemy's most powerful weapon was the best way to go or not.