You spent time on their stuff that day instead of working on something else or some other client. The opportunity cost to you is the same, whether you leave at 3pm or work until midnight. Charging by the day or week allows both you and the client to not think about low level implementation details like that.
The inverse may be true however. If you skip out 1 hour early every day but still accomplish a lot in your opinion, the company may feel that if you had stayed that one extra hour each day, over a week's time you billed them for a full day that they never received.
They may argue you could have accomplished the task 1 day early, saving them $X, etc.
I agree with your value proposition -- but I feel it's difficult to pull off in reality. I feel a lot more comfortable with billing Time and Materials rates, so basically if I skip out early one day, they aren't being billed for it, and the project goes on at it's pace.
