If you click on the link below to my Anime Expo 2007 photos, you'll see lots of pictures of people in costume. That does not mean that everyone who attended the Expo wore a costume. I, for instance, did not. Here are some photos to prove it.
I am, however, wearing a motorcycle and a big fancy Hellsing chair. I don't use either in real life. But those aren't costumes. They're props. There's a qualitative difference.