If you know you are never going to use host specific variables, like you already know what the hostnames of your machines are, don't need to dynamically figure out their IP addresses for use in templates, and already know the OS, chances are you'd like to skip the time consuming call to the setup module. It's a small amount of time, but it's there.
Further, if you have a very large number of systems and are still using pull mode (which is fine), you may wish to turn off fact gathering so ansible's RAM requirements remain small.
To do this, at the top level of any play, you can now do:
gather_facts: False
Works as advertised, the default is True, meaning no playbook changes are required to keep ansible working like it already does.
--Michael