I think it has something to do with the muscles retaining water as they repair themselves, so having that rest day would have a more noticeable drop on the scale as the water is expelled. In my uneducated opinion.

sounds good though.
I think rest days are vital for each muscle group but not necessarily for the whole body. As in, it's good to exercise every day as long as you're not doing the exact same thing over and over. Just think, back in the day when life was all about farming and other physical labour (or today's construction workers etc), there was no need to rest because our bodies are built to handle the workload.
edit: but i certainly don't mind when the weekend roles around after I've been thoroughly consistent with my exercising all week. I tend to take 1 or 2 days off then.