A lot of the "myth" about barrel break-in stems from the fact that the process was developed back when most barrels were broached, not button-rifled. With a broach-cut barrel, you would fire and clean in a sequence until you had no more copper shavings visible on the patch, typically 25-50 rounds. By doing so, you would burnish the barrel and remove any slight burrs left from the rifling process. Today, most barrels are either button-rifled or hammer-forged, neither of which will take much to "break in", possibly 10-20 rounds max, as all you are trying to do is smooth out the leades.
So, is barrel break-in a myth? Not in my opinion, when it was developed it was a real benefit. Today, with hand-lapped barrels, it is just another one of those things that is becoming obsolete (like cleaning your rifle right after shooting it) and will soon go the way of the dodo.