It was an interesting thread, but I've noticed a number of people getting in a snit saying they will wear their training t-shirt wherever they please. Well, the thread topic specifically states
wearing your gi and belt in public. It doesn't say anything about t-shirts. The original post was about a person wearing their full karate uniform in public. My personal take (by way of my senior Japanese instructors) on such a thing is that it shouldn't be done. Not for self-defense reasons, and not for fear of getting your uniform dirty, but because the uniform is for a specific purpose not shared by most of today's society, therefore you will stand out like a flashing neon sign. This is the antithesis of traditional Japanese martial arts training, where a person is supposed to avoid drawing undue attention to themselves through anything other than the beauty and effectiveness of their movements.
This outlook is why the traditional Japanese martial arts tend to disassociate themselves from the more modern "karate" and XMA schools with their flashy weapon katas and bright sparkly uniforms. As one of my instructors put it many years ago when asked this very question ... "we are not a circus, so don't make yourself look like a clown!"
Just my opinions, worth what you paid for them.