Regarding the regularly appearing statements about how the AR already has fast takedown, and you could buy numerous uppers for your barrels, that is true, but has its limits.
If you want to have field-configurable options, it is simply not feasible to be carrying extra compete uppers around. Also, it gets very expensive if you want to have nice free-float tubes or other accessories on all of them.
There's no getting around it, the MGI system provides more versatility for configurations and calibers, with less carry weight and less bulk, and also costs less when you factor in all the things you'd need to have for each separate upper you'd want.
If you want very lightweight, but high-performance barrels to go into your MGI QCB, so that the barrels you carry won't weigh you down either, then just take a look at the ABS carbon fiber barrels that I offer on the EE.
This is a pretty enticing combination, even if I do say so myself, and I own the combination myself for my personal weapon.