Did you spend the last 20 years in a coma? Anyway, first it was about quickly changing the calibre, but since both militaries and civilians realized that it's way too much of a hassle to change the cartridge fired by an individual gun. Now it's about changing the barrel lenght. From a technical standpoint, hunting rifles already proved that you can make a rifle perfectly accurate even if it's held to the receiver by just a single bolt (the threaded kind). Especially if the bolt locks into the barrel, which is how it workson weapons based on the AR-15 and AR-18. And indeed, it's simpler for the manufacturer too, because you don't need a to trust your workers to thread in the barrel the right way, they just have to quickly attach it to the weapon and the zero the sights.
Does anybody have some first hand experience with .357 Maximum? Is it just a bit stronger .357 Magnum, or is it a significant jump, like e.g. going from .44 Magnum to .454 Casull?