An interview candidate showing up in a T-shirt is a serious black mark, not because I care about how they dress at work, but because they don't care enough about the interview to put on a shirt with buttons and a pair of dress shoes.

If somebody has that level of respect for me, the company, or the whole song-and-dance process, what are they going to be like to work with? What happens when we need them to deal with customers?

Sure, they may be an amazing coder, but a bit of politeness and some social graces make for a better working environment any day.