I am replying very late to this thread, and probably most of you have lost interest after six pages of (as Ken says) "religious" argument. But I'd still like to comment.
The long version of what I have to say is on portafilter.net.
Here's a shorter version:
To me, weighing espresso is very important when you are communicating about your espresso to others. By weighing the dose and weighing the beverage, you can accurately describe the kind of shot you are pulling to people who may live thousands of miles away from you.
This point was emphasized to me in a two recent email conversations. Ken Fox and I were talking about weighing shots. He replied that his doses were mostly in the 14.5 to 15g range, and the shots weighed from 9.8g to 14.5g. This surprised me, because although I knew Ken was a low doser, I didn't know he was a ristretto puller, too. If he ever sends me more samples of his coffee roasts, I will start out pulling them in a ristretto style. When he previously has sent me coffee, I have pulled it more lungo.
Doing it ristretto
instead, I will be more likely to "get" what he's trying to do with his roasting.
Another friend told me of his visit to a famous espresso machine factory in Italy. In their testing lab he measured their doses at 14-17g and the beverage weights at 54-60g! That really surprised me; I can't imagine pulling good shots that long with the beans I use and my setup. Sometime I'll get some decent Italian espresso beans, lower my boiler temp, and try it out.
An additional reason to weigh shots was pointed out in a pretty brilliant post by James Hoffmann.
Read it for yourself, but the gist of it is, you can often dial in an espresso by sticking with a fixed brewing ratio (in James' case, dose g/beverage g = 65%) and adjusting grind to taste. Irish barista champ Colin Harmon
made a similar observation on his blog.
Like many other commenters on this thread, I don't think one should be in the habit of pulling every
shot on a scale and stopping the pump on every
shot based on weight. It is very valuable, IMO, to develop one's skill in assessing the flow rate, texture and color of the espresso stream and ending the shot based on those cues. If you're always watching the scale rather than the stream, you don't develop that skill.
So each technique has its place. To the extent that we can leave the religion out of it, I believe we can help ourselves and each other to make better espresso.