The whole removing metal thing is really about the edges of the rifling. We are running a compound, patch, etc., over what is supposed to be a precision edge. Doing that is going to have some negative impact on the edge, just like if you did it to a knife. The other concern is using too much compound, or using compound in the wrong way, is negating the lapping that has already been done to a precision barrel - obviously less of a concern on an unlapped barrel.
The counter is that all this stuff is less abrasive than the bullet screaming down the pipe. Even the "rub it on plastic and you will see it does not scratch" is garbage. I myself have done this to clear up clouded lenses. Yes it did not leave visible scratches, but it did remove stuff. Over time the "clear" goes away and you can see where the compound stripped away a fine layer of material. The no visible scratches is a human eye thing. It scratches plenty, just not that we can see. If not, how the heck would a compound be any better than liquid?
Fire lapping on the other hand, that is taking material away in a measurable way. Even Tubbs' instructions for his cleaning bullets say the process removes up to 0.0003 inches of material. Is that a lot? maybe, maybe not, but it is removing material.
The barrel manufactures don't help me much either. They give the list of do's and don'ts, but with all the distortion we Internet scholars put out there it is hard to say of the manufactures lists are because it matters or because they don't want people doing stuff then blaming them for a bad barrel. I for one would not blame them if it is for the latter.
I am no engineer when it comes to this stuff but I do think something that attracts more bullet jacket or constricts the movement of the bullet is negative that keeps getting worse until you do something about it. At that point we are likely trading temporary relief for a less perfect bullet travel down clean grooves.
