I think I may come from a different perspective from you, or indeed many people here. I probably should have pre-faced with this first.

While I code every day I also part BA. I have a lot of contact with non-technical people - I am not just in a big office with other members of my profession.

Basically if I'm talking to people wearing suits/business casual, and I'm wearing a t-shirts and shorts, they will look down on me. You may decry the arbitrariness of social norms but they do exist and they do effect your interaction in a professional setting, whether people in silicon valley think they should or not.

I want to be respected and I want my field to be respected. If we dress much worse than other professionals, we send the message that we're not really on their level.