Recently I've read a very interesting post by Anna Forss called "Stupidity of the Team". While Anna concludes, that it's healthy to introduce diverse opinions and invite opposing minds to dissolve the like-mindedness of homogeneous teams, I think there's one important nuance that shouldn't be overlooked.
Let's think: teams exist for some purpose. To resolve some goals. If it's a small product development company - then this team exists to develop a product. Permanent rebels are not welcome in any group - because what they do with their rebels, argues, drawing attention to themselves - they blur the focus of the whole purpose why team exists. Of course, a team will naturally outcast this person. Next, if a team is bombarded by controversial opinions and judgments, they will spend all their time evaluating and thinking if this is right or wrong. They will get busy sticking tags on new opinions instead of focusing on their work - and they will inevitably lose their focus.
Life in a small development team can be compared to living in a sheltered reality, with it's particular culture. An isolated sheltered reality will not last for long if it's completely isolated, so emerging on the surface for a gulp of fresh air is really needed. As a member of a small team - can you remember when the opposed rebels and opinions really did help? When they triggered something that the team would not have thought by themselves? Well, of course, if someone comes up and says - "your UI is bad" - then another person comes up and says - "your UI is bad" - then you start thinking that it's indeed something wrong with it. You've got this signal from outside world. You work on it. Basically, you know what you should work on. The outsider's opinion has accomplished it's task - the outsider's opinion can now go, because you're not interested in hearing variations of one and the same opinion. You get to work, and you work to develop a nice new UI.
There's no need to focus on outsider's opinions and pay too much attention to them. Outsider's opinion is just a trigger to team's actions - it's not something that the team should busy their brains with all the time. In a way, diversity of opinions may be even harmful. I guess that's why we've got leaders - authorities who tell the crowd "THIS is your Holy Grail".
My conclusion is: healthy vaccination with opinions opposing a team's culture is good. But don't overdo with them. Too many opinions will not increase collective intelligence for this team's specific purpose, they will blur it.