I have a very similar setup as the one mentioned in the article where we create a full baked ami, then create a new autoscaling group which attaches itself with a load balancer.

The reason you want a fully created ami is to greatly cut down on the time it takes to accept new traffic. Lets say you get a spike in traffic, if you have a fully baked ami your autoscaling group can get an alert of the increase in traffic, launch a new instance and that instance will start handling traffic around 60 or so seconds. If you booted a generic ami and then provisioned the box you could easily be looking at 10's of minutes before it could handle traffic depending on the complexity of your app. This could easily lead towards long periods of down time.