I disagree that it's the wrong metric. It just shouldn't be the only metric. There's a natural entropy that occurs in the workplace that seems to decrease the real work time that is available. It's made of up meetings that you don't really need (some you do), admin overhead, blog reading, SSC reading, water cooler talk, task switching, etc. From time to time it's useful to look at where you time goes and ask yourself (or your team) are those things in the right proportion? Are we doing enough 'real' work?

As a manager I know that you can't get eight hours of real work in eight hours. My goal has been 6-7 hours a day, where 6.5 is the sweet spot and 7 is laser focus. That leaves an hour a day for some chat, a meeting or two, and all the other stuff that creeps in. Now in that 6.5 hours I might do one task or ten, write thirty lines of code or three hundred, but it's time when I am focused on whatever real task(s) I needed to be done.

It's easy as an employee to say that you don't want to be measure on hours of productivity but only on tasks completed. The problem with that is that in general you're paid based on hours. Imagine hiring a plumber at $75/hour and watching them stop to answer a few instant messages, or knock out a blog post because they just learned something really cool while fixing your problem. Would you have a problemw with that? Yeah! On the other hand, if you were paying them a flat fee to fix it, you'd be a lot more tolerant of the time it took to get done, other than if it turned into all day and you had to sit there with them.