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.
DevOps here! Very much this. You can use Jenkins with Packer Builder and a configuration management tool of your choice to build an AMI for each code deployment (AMI generation costs nothing except for the space of the AMI). Need to scale 100 instances in <5 minutes? EASY.
What you don't want to do is attempt to scale, and you can't reach Github, or you overwhelm your configuration management tool, etc. Your AMIs should be built so they need almost nothing from external resources before being ready to serve requests.
Totally agree. In practice, we've found that running bootstrap scripts on fresh instances is a major pain point. When an instance is spun up as a result of a load spike, you need it up ASAP so waiting for bootstrapping to finish sucks. It's really a tradeoff between flexibility (configuration in code) vs spin up speed (configuration baked in AMI).
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.