Lightweight is good. Our IDPA club runs carbine matches once in a while. Where you're running and shooting at targets at 10-50 yards, a light carbine is good. If this carbine is for home defense (under 15 yards), then what you're doing is not just a novelty. In this case, MOA at 200 yards is nothing more than tacticool talk.
The idea is that you are more likely to have a Lightweight gun with you when you need it if you are carrying it.
Also, since it is my wife's gun, she ain't manhandling a 8lb battlerifle around and it generally is only going to have to work for 1-3 magazines. heck, in a home defense situation, one 30rd mag will most likely take care of the problem. It is superfast handling.