Really, how so? If I can simply down vote to disagree why should i be bothered to make a comment?
People can do this now. Yet discussions are still happening so I don't think a "I Disagree" option will change that.
They can but they shouldn't. I still believe that even if the current down vote button is used as a "I Disagree" button not all do so and use it as it is supposed to be used.
I think in the future more people who would have commented simply use the "I Disagree" button. Hopefully that will not happen, we will see.