"Peak pressure occurs when an accelerating rifle bullet has gone maybe three inches or so. Therefore, we need not be concerned about choosing a different burn rate by barrel length;"
Yes, the max pressure is reached when the bulled is just a few inches down the tube. But the powder is still burning and producing more gasses the whole time the bullet is in the barrel. The bullet will increase velocity for every addition inch it travels while the powder is still burning. The longer the powder burns,faster the bullet will be moving.
And for rifle shooting, I agree that selecting one (rifle) powder over another (rifle) powder just for its burn rate is prolly not very efficient use of your time!
But, burn rate is very important when you load for pistols and carbines too. Example, I load 44mag for my carbine with slower powder, than when I load for my pistol. The pistol load needs to burn as much powder as it can in the short barrel, so it needs to burn faster than a rifle powder.