I’d like to see online communities where people with a diverse range of opinions are able to interact in a productive way. This is for the reason that @hellekin mentioned. We live in a world where people have a diverse range of opinions. Somehow or other the online world should facilitate discussions among people who disagree about emotionally charged issues.
For communities that are focused around interests, probably most members need to share some basic assumptions. For example, too many atheists would probably be disruptive to a community focused on bible study, but having a few atheists in the community might be seen as an interesting challenge.
Even for the case of communities where most members share some basic assumptions, having a diverse range of (for lack of a better term) personality types is probably beneficial.