My response is this: it can't hurt, and it seems to have some beneficial effects.
I have never met a shooter that can tell the difference in shooting a gun whether it had "barrel break-in" or not.
We have compared the accuracy of two AR15s (Colts, same M4) where the other guy broke-in his with whatever the magic formula dejour was and I didn't. I fired approximately 300 rounds while he did his 100+ round barrel break-in. I also checked the game cameras, did some target maintenance, checked my feeder, etc. waiting for him to get through all his stages of cleaning. Shooting the same ammo, we can't tell the much difference between the guns. If barrel break-in improved his gun, then it seemed to make it on par with mine. They both shoot about 1 MOA and on a really calm day, maybe 0.8 (and after we put ACOGs on each).
I realize that two guns doesn't make for a significant scientific test, but they were two models of the exact same gun, bought within a couple of weeks of each other. I know haven't seen any documentation to prove that break-in actually does anything for the gun over just shooting it. I certainly haven't seen any such work with a statistically significant sample size to show the benefits of barrel break-in. Nor I have I seen any study that given the proper procedures for an optimized barrel break-in. People who do it all seem to have their mystical pseudo-religious beliefs about how many rounds between cleanings, staggered or equally spaced sets of shots, and some think you need to wait X amount of time between every shot so that the break-in is with cold bore shots (which takes forever to break-in a gun), and what cleaners to use at what points in the process.
They all have these idiosyncratic break-in methods, but thusfar I haven't found a single person who can justify why their particular method works over any other method or that can show me anything diagnostic to substantiate that barrel break-in improves the performance of the barrel. That is why I call it pseudo-religious as all of the beliefs around barrel break-in seemingly are based on faith.
I cleaned the gun after 300 rounds. I only brought 300 rounds to the range. I would have shot more, but ran out - hence doing all that other stuff. He cleaned his something like 15 times during the first 200 rounds and then thoroughly afterwards. Both guns are broken-in and fire comparably. I see no difference for all the time and work he put into his gun.
With that said, I am not totally closed-minded to the issue. If somebody out there can point me toward a source/article that can show how these complicated break-in methods do actually benefit the barrel and improve performance over not doing them, I would very much like to learn more about the process and just exactly how it works.
Too vigorous and improper cleaning can do damage to the barrel ! Shoot it , clean carefully and enjoy !
And that pretty much says it.