I just did some consulting on helping another team improve their hadoop cluster performance and the first thing I noticed is that all 40 boxes in the cluster were burning a CPU core with a puppet agent process that was running at 100% CPU for months.
That's one of the nicer things about the no agent setup, when Ansible is not managing something, there is nothing eating CPU or RAM, and you don't have a problem with your agents falling over (SSHd is rock solid), so you get out of the 'managing the management' problem as well as the 'management is effecting my workload performance' problem.
In particular with virtualization, running a heavy agent on every instance can add up. (reports of the ruby virtual machine eating 400MB occasionally occur).
