I'm not sure that 'identity' can carry all the weight for why 'non-academic' discussions devolve into arguments. Unless you mean identity with the group and not some nefarious identity that is contained within the individual's psyche.
I'm thinking that people sometimes enjoy being right about something. They enjoy having an opinion that has merit and that can benefit others. Sometimes expressing an opinion can be just to 'one-up' the guy who is castigating some poor bloke in a forum who failed to articulate his position well and who is now being bullied. Sometimes the expression of an opinion can be meant to come to the defense of another. Perhaps it is those unspoken elements of human communication, the binding and unbinding of groups, that are being transgressed, or which we mistakenly believe so, and not our sense of identity, which gradually raises one's ire when their opinion is being criticized, tested or what have you.
One of the reasons we communicate our positions is to ally ourselves with others. We find common ground and can then feel apart of. People get upset when they risk losing this common ground. When the matter is not so crucial, such as the preference for an x or y programming language, their chances of losing affinity with the group is less and so they tend to be more easy going about the expression and consideration of their /preferences. That's my 2 cents :)