I think the purpose of this ratio is that a predictable low is better than an unpredictable high. It's sort of the same idea as padding out an ECD. Better to sometimes impress than sometimes disappoint.
It is better. If you have a predictable low rate other people can still plan their schedules around you, while you figure out how to speed up / scale up. If you have an unpredictable rate, you block everyone else (and often yourself).
That's a fair criticism for certain contexts. Most people I've worked with desire to do well and to feel good about their accomplishments. In those sorts of teams, this works. It would likely not work as well in an environment where people were just hoping to do the minimum possible.
Wouldn't you prefer (committed + 1) / completed? Or (committed + 2) / (completed + 1)?
In the limit, you want this to approach 1 (namely, you complete more or less everything you commit). A bad situation is where you're overcommitted and have many committed things, but few completed ones (e.g. 10/2), which results in a high ratio. But, with your proposed metric, you'd achieve the asymptotically ideal ratio with 2 committed projects and only 1 completed one, and in fact the global optimum is "I promised nothing and did nothing, yet achieved a ratio of 50%".
Haha, you're right, I meant `(completed + 1)/(committed + 2)`. A value close to 1 means you completed a lot of what you committed, and a value close to zero means you completed a very little. I stole it from Laplace's rule of succession.