Helotaxi, I respectfully disagree.
While you are right that the bulk of the powder gets burned in the first few inches of a barrel, a shorter barrel will leave more powder unburned or not fully burned than a longer one. This is obviously more marked with slow-burning powders such as 4350, 4831 or 450, with cartridges that require a heavier charge, such as the Wby Mags, and with lower rather than higher pressures.
Here's a quote from a S.W.A.T magazine article:
According to Mike Rescigno, President of Tac Ops, the 22-inch barrel is ideal for the tactical shooters that are going to use the 190-grain Federal Match ammo. There isn't any loss of performance by going to the 22-inch barrel and this round. The Alpha 66 still provides 1/4-MOA or better accuracy.
For heavier bullets or hotter loads with slower burning powders, Rescigno recommends a 24- to 26-inch barrel. The longer barrel length is necessary for complete powder combustion with these loads. (Emphasis mine)
This also bears out with my experience working with engineers of rifle manufacturers, when we would discuss introducing a new hunting rifle in a "hot" caliber--complete combustion of slower-burning powders was always a concern.
I do agree with you that this is largely an academic discussion, though--there are a couple barrel lengths that are a commonsense minimum for standard and magnum cartridges respectively (well, depending on bore size too, but now we are wading into Megageek Land), and as long as we stay within range of these lengths, everything else is superfluous.
Please let me know if there is more recent data that shows otherwise--I'd be interested in seeing that, also because I've been out of the loop for a few years.