Loss of momentum. When I'm coding, and I'm on a tear, I generally won't stop for drinks, food, or any other outside distractions. It's this exact reason that I abhor the cubicle culture so much, as it tends to invite distractions, generally when I'm in the midst of my greatest productivity.
If I'm on a roll with something, I don't want to chance that I'll come back from a three day weekend every week and have to remember where I left off, or spend that time trying to get back in the groove.
One of the best places I worked (now defunct) allowed for you to work hours per week -- if you worked over, you were credited it for the next week. It was great to crank out marathon sessions, and then, between projects, you could take the remainder of the time off, to rest. So if I spent 7 days straight on a project, I could take 4 days to rest and be 'neutral' on my schedule balance (assuming 8 hour days).