I'm all for it when used correctly. Kroil is amazing stuff and that's my primary cleaning solvent unless I'm trying to get copper fouling out.
The JB Bore only comes out if I have a carbon ring that I can't get out after soaking the barrel over night with the Kroil. I would rather use JB Bore (properly!) than a standard bronze brush in a high end barrel.
The JB Bore also works for trying to save the throat on a barrel that's been shot almost to the point of no return. Then there are those times when no amount of lapping will save the barrel. (RIP 300 RUM barrel

)
Andrew