I have a friend who bills by the day, because it gives _more_ flexibility and not less. His explanation: sometimes you have days where you only put in 2 hours,

I have a friend who bills by the day, because it gives _more_ flexibility and not less. His explanation: sometimes you have days where you only put in 2 hours, other times you have days where you put in 12. He chops a project up into how many "days" he thinks it will take and runs with it. Really the "days" becomes an arbitrary unit of measurement at that point, which is really what "hours" are too but it's hard to sell that to clients sometimes.