This is an excellent thread.
When I started to cook bbq about 11 years ago, I had a few mentors.
I started by using a WSM.
One of them was quite anal about using only 225 degree temperature.
To the point that if it didn't get to that temp he was slightly lost and panicked.
I thought it was excessive but I did it and it helped me learn how to control my fire.
I believe that having a certain acceptable range for different meats is the logical thing to do.
I think most people try to keep their temps relatively low for ribs.
For briskets and pork butts I have had good results with different temps all over the map with high temps working quite well.
For someone just starting out, guidelines will at least give them some sense of control.
The key to know is that the best cooks are always experimenting and what worked last time won't necessarily work this time.
With experience I have found that times and temperatures can vary greatly especially with briskets.
Every piece of meat is a different animal and some may have tougher muscles or more fat or less fat much like humans lol.
Every piece of brisket needs to be approached on its own as if you are doing it for the first time and yes, it is done when it is done.
I think what some may worry about opening up the smoker too often to poke and check it if is done by feeling. Probably afraid that you will lose temperature control or maybe get a dry product by opening it too often.
Again, have a certain guideline may give you a range that you think it is done.
Nailing it at just the right time to take it out of the smoker along with the proper resting time with precision is not going to be found by reading a thermometer, it is going to be done with your own experience.
That my friends is a much more valuable tool than a 10K dollar smoker, an expensive thermometer or a rolex watch to get your times right.