The whole "number" thing is somewhat foolish. It's a good guide especially for those who aren't financially responsible but for most of us here and on other financial interest forums somewhat useless. A very, very large majority of people's "numbers" are much higher than what their actual expenditures are, or will be in retirement. I am also guilty with estimating more than I'll need also but i guess that's not a bad thing.
You can't manage what you don't measure, so what other metric would you propose? It's great to have an easy to remember goal and to be able to look forward to attaining and surpassing it.
Even when you're dieting, you're either measuring the number on a scale, the number on the inside of your pants, or the number of pushups you can do. Numbers are awesome tools as long as you don't get too hung up on any one of them.