I am not discounting the use of the recirculation button completely - it is very useful for cooling and heating quickly, initially. Over time though, the humidity is going to build up to very high levels, which is why you should usually use the outside air setting.
As further proof, the defrost mode will not let you use recirculation, because humid air would be blowing up on to your cold windows and condensing. Maybe not at first, but after a while...
