And yet, people aerate by splashing, shaking, and stirring all the time. The air in the headspace of your fermenter is just that, unfiltered room air. One of the most popular starter methods is "shaken, not stirred", which incorporates as much regular old air as possible by shaking. Once you get to the point of pitching enough healthy yeast for a starter or a given batch size, it's going to out compete any stray microbes in the air.
I use an inline filter as cheap insurance for aerating but if I lost it I wouldn't think twice about it.