I'm not that familiar with Fabric, so maybe it can do some of the things I mention here out of the box without a lot of custom code. I'm only familiar with Capistrano and I associate them in my head, perhaps naively.
Your example is great to illustrate why configuration management is so powerful.
Most importantly, your example will upgrade the package to the latest version every time you run it (which is probably not what you intended, but maybe it is). I'm sure there is an idempotent command for each of apt, yum, ports, homebrew, etc, but I have better things to do with my time than figure that all out.
In puppet your command would be:
package { 'foo': ensure => installed } # or 'latest'
What if you also have RedHat servers? Now you need to figure out the right yum command to use to perform the equivalent. Or your developers Macs? Homebrew. Chef/Puppet are aware of the environment they run in and install appropriate packages using the appropriate package management script with minimal alteration to the underlying recipe.
For RedHat systems, it would be:
package {'foo' : ensure => installed }
On a Mac, it would be:
package {'foo' : ensure => installed }
On that Arch Linux machine that your devs are running, it would be:
package {'foo' : ensure => installed }
Now, let's throw some dynamism into the mix...
What if you want to build an haproxy configuration file that pulls the list of hosts from an authoritative source? Do you "just" build it up from scratch every time and replace the file on every run of your script? With chef/puppet, you can pull that information in from your node classifier using the same query for dev, staging, and production and have the same recipe work for multiple environments. In some environments, the haproxy config has now changed, so I want to tell haproxy to reload. But in other environments, the config stayed the same, so I don't want it to reload. The file and service don't get touched if they didn't change.
What if a service has multiple files associated with it that when one or multiple of them change, you have to restart or reload the service? You don't want to restart each service every time one of the multiple files changes, so you end up having to build a queue to handle the service notifications.
Say I've got a few dozen users on my hosts and someone leaves. Which is easier - writing a purge script which knows how to purge the user from all environments that you support, or this...
user { 'joe' :
...
ensure => absent
...
}
I'm certain that that you
can do these things with fabric scripts, but by the time you have, you've either written your own configuration management tool (which is unlikely to be as robust as chef or puppet) or you have a hodgepodge of unmaintainable scripts.