Productivity is a tricky word in case of programming. The usual definition is how much you produced and also the produced thing should be tangible in some form or other but we all know that good programmers spent a lot of time thinking about things, trying various alternatives etc until they make a particular decision and then implement the decision. In such cases it becomes very hard to justify the productivity. Also when I am working on some particular problem, the problem is always with me in my mind. Whenever I get some free time I tend to think about the problem, no matter where I am or if I am at office or not. This leads to this idea of "working for fixed hours" being dumb in such cases.

Of course this doesn't apply to all programming jobs, specially where you are given a requirement and you just go ahead and implement it without thinking too much about the big picture of the whole software system.

A thought can come anywhere anytime, it doesn't care if you are in working hours or not :)