I was costumed once as running crew, and had a near-nasty accident because of the costume. It was decided that since the costumes were for the crew, they didn't need to put any effort into them. I ended up in a skirt that was literally four sizes too big for me and had to be wrapped around my waist and pinned. I had to run backwards during one scene change and my shoe got caught up in the skirt fabric, I went down on my butt and thanks to our momentum, went 'skipping on my butt' across the stage. While it was amusing to everyone else later, I was too busy at the time trying to stop the heavy table leg from dropping and crushing my neck.
Now, this might indicate that I am totally against crew in costume - which I'm not, but it would take a good reason why they need to be in costume to get my full support; as well as the costume taking the crew's needed movements into consideration. I worked in a union house once where a hand was needed center stage to open up a set piece that rolled on, so they worked it out that one of the hands wore a costume and entered from inside the piece after it was wheeled on. In that circumstance, it worked out.
BTW - he was also paid extra for wearing a costume, per the theatre's contract.
-Centaura