Because dialog happens with more then two people, and like what mr kevinbl00 proves by commenting here is that conversation between two parties who "can't get along" is a useful and valuable thing to have. Lets take a moment to remember that the comment below only exists because of my refusal to mute people. I, meanwhile, am unable to respond to this person because they have me muted. Is the comment below valuable, useful, and an overall positive thing to have in the community? Absolutely. Would my response be, if I were able to make one? I certainly hope so. By muting you remove that potential, and any time two people disagree in a way that is not reconcilable hubski will only serve to have those two parties mute and ignore each other, rather than discussing and coming to any form of understanding. "But people did it in the past" is an anecdote and a useless one at that. You have no knowledge of how many useful comments, of how many great points have been silenced thanks to this mute future. You only see the "people who learn to get along" which is not going to be the trend, if my experience with human interactions is anything to go by. Your system shouldn't rely on choice.why would you want to comment on or have a dialog with someone that clearly doesn't want to have one with you?
I have never muted someone because I didn't like their opinion, but I have muted a number of people because they were either overt spammers or complete assholes.