Great essay and discussion.
I think there's another variable strongly influencing how "identity" behaves in the wild.
Consider the examples of Nelson Mandela and Ghandi. Both men very strongly identified with the cause of their people, but despite that identity had an amazingly noble approach to their oppressors. Horrible atrocities were committed by their foes, yet they reached out to them with grace - both during their struggles and after their victories. They studied their "enemy's" culture and language extensively and had empathy and compassion for them even while fighting for their cause.
There are many examples of wonderful Lincoln-esque people who strongly identified with a cause, yet had compassion and understanding for the other side. From examining them, it seems that identity was not a factor in causing strife and stupidity when it was coupled with that genuine graciousness and understanding.
Tragically, there seem to be far more examples in history of people taking the other approach - envy, hate, violence, etc. In those instances, identity seems to be the vigorous fan that stokes the flames.
While those may be extreme examples, the same principles apply to today's conversations. For instance, very few people would get into a heated debate with a down-syndrome child over a religious point of doctrine or political policy - no matter how adamant she was about it. Why? Well, it's hard not to have compassion for a mentally-handicapped child who is so innocent in her understanding. Yet when people come across a 35 year old man with the same view (who incidentally may be just as 'innocent' in his understanding) - it's much easier to throw away that compassion and understanding and attack him personally with vitriol.
If someone has genuine love and concern for another person it's hard to imagine the kind of stupid debates and spiraling into dumbness that pg describes - even if the participants are at polar ends of the religious/political identity scales. Some of you may remember what ParticleTree did to effectively reduce trolls and flamers: http://credibility.stanford.edu/captology/notebook/archives....
By giving commenters a reminder of compassion, they took a lot of heat out of identity-stupidity.
Lincoln, Ghandi, Mandela etc understood this principle and used it for great good. It reminds me of a great quote from one of my favorite sci-fi novels that shows Charity not as a flaccid quality - but like this:
"And now it came. It was fiery, sharp, bright, and ruthless, ready to kill, ready to die, outspeeding light; it was Charity, not as mortals imagine it, not even as it has been humanised for them since the Incarnation of the Word, but the trans-lunary virtue, fallen upon them direct from the Third Heaven, unmitigated. They were blinded, scorched. They thought it would burn their bones. They could not bear that it should continue. They could not bear that it should cease." - That Hideous Strength, CS Lewis
I've know highly religious/political people who were extremely dedicated to their views, yet seemed to get along with just about everyone even while having lively debate that even decreased dumbness! Unfortunately, we've also seen much of the opposite. The main key always seems to be whether the person has genuine love and compassion for the others holding opposing views - so much so that the others know that disagreement doesn't threaten the relationship. It's a rare and wonderful quality we can all strive for.
In a nutshell - have all the identity you want - just make sure you couple it with the Golden Rule (thankfully found in just about every belief system, religious or not, yay!).