The problem with cluster SSH is it's not declarative, so if one server already has a package, and another does not, your servers will not behave the same if typing into all of them.

This is why Ansible modules (and most other config tools) instead describe the desired end state, and perform the commands needed to get you to those states.

Basically all config tools arose out of a need for solutions to replace parallel scripting with something more robust that accounted for potential variance between systems, and also to make things more push-button.

I can see that for some kinds of admin tasks, but why package management? At least on Debian, the package manager is already declarative: There's an install/upgrade planner that figures out how to get from the current system state to the final desired state, given constraints such as packages' declared Requires/Conflicts/etc. lines. If the system is already at that state, it does nothing.

How often is setting up a package just running apt-get? There is an overwhelming majority of times where you have to change something in the config file somewhere. Ansible can do anything you want to provision your boxes.

Indeed, cssh is great for getting a quick visual on a few servers, but you're pretty much limited to what will fit on your screen. Ansible really shines when you scale it up a bit further, and it grows well, allowing you to add bits and pieces as needed.

The hosts/inventory in ansible as a source of truth for your infrastructure is also a nice feature. You could use a selection from your ansible inventory to open a cssh with 'top' running on all your webservers, for example.