What does hosting with Heroku have to do with using a trigger?
> On a related note, it feels good to know that everything our app needs to run is in the code base.
Except the database schema and any additional indexes you need to make it not perform terribly. All basic setup, just like creating triggers.
> Rails fights that with migrations and the callback chain too, so I guess that thinking has sunk in for me.
The problem with doing stuff like this in a callback is that an additional query is sent to the database which can be a big performance problem if the insert load is high. I generally agree that complex logic should be avoided in triggers and is better left in the application code in most cases but incrementing a counter is about as simplistic as you can get.
