Why? With clients you wear more formal clothes for PR purposes. It shouldn't be necessary on most dev teams (in my experience).
I think the same applies in the given example: the in-group knows the guy in the shorts and T-shirt is a nice dude who helps others and is super-professional about quality. The customer, as an outsider, just sees someone who's underdressed.
1) company that creates custom modules for firms coal software; it always involves meeting the customer. 2) Open office layouts where customers are given a tour of the Office.
On the other hand, at one place I worked it would have been illegal for the customer to interact with us in our office space, but we still had to wear formal office clothing.
Consider
Case A: Customer knows you well understands you are competent. If you want to leave or are laid off that customer might eityher hire you or know someone who does. Or at least will give you a solid reference.
Compare with.
Case B: Customers and others only know you through your boss.
Not to mention having to hear the same old jokes every time the customers go look at the monkeys.