I'm going to disagree here. WN does exactly this, basically, and the boarding there is considerably more efficient that an AA aircraft of similar size. I can't even tell you why....just seems that people are more considerate about getting out of the aisle to let people pass...but it has been my fairly uniform experience.
Cheers.
But on WN, there are no assigned seats, so everyone just takes one. Doing this on AA would be chaos - how many times on AA do you see someone who somehow walked past their seat, and have to swim against the stream to get back to it? This usually adds a good 5 minutes to the boarding process, even with the boarding staged correctly.