Don't you know you're supposed to look business appropriate at all times in case a customer sees you outside of work ? You NEVER want to give a customer the impression that you are not professional at all times!
I just took a class on how to interview for a job, and that's one of the things the teacher said; to always be dressed in business wear and always be carrying your resumes with you just in case you run into someone who may be hiring.
Look, I need a job, I really do, and I've been really working my ass off to find one, but I strongly doubt I'm going to run into my future boss at the grocery store. And even if I do, if he/she's so anal-retentive that I'm expected to be dressed to the nines to buy cereal and dish soap, I'm not sure I'd want to work for that kind of person anyway!
My time is my time. Not the company's time. If I want to wear jeans and T-shirt to go grocery shopping, or to take my son to the playground, or wherever, I damned well will! I'm not wearing out my good clothes or dress shoes when I'm not at work. Funds are tight!