We see that at the club all the time and I don't get it. It never ceases to amaze me what some people will wear out. It is especially perplexing to me given the fact that they are going to try and meet people with the expectation that the people will find them attractive enough to have sex with them.
We allways dress up, even if we are just going over to friends to have dinner. I would do it if for no other reason than it improves my self confidence if I am dressed up a little bit.
As far as sweats go, they are the anti-viagra in my opinion, I've noticed that their are very few women that don't look repulsive in them. Sweats are outlawed at our house, ick!