Well, if you ride on dusty roads or encounter a sand storm... you're going to be pulling abrasive into your cylinders. This is obviously not a great situation.
Race bikes run for relatively short times before being rebuilt, noise levels are allowed that would be prohibited on public highways (although we all know how such rules are ignored), and racetracks are relatively clean compared to many public roads.
The screens keep out larger stones and such. They don't filter out dust. They also help extinguish flames from a carb backfire, having flames shooting out under your fuel tank is unadvisable.
Current designs on sportbikes have a large airbox with what are effectively velocity stacks inside, some have variable length systems to tune the engine for power at differing RPMs even. The stock 400F system isn't really like this, and is basically a bit of plumbing to take air from the filter to the carbs... but the carbs have been set up for the resulting airflow. The stock filter and particularly the airbox intake snorkel limit flow considerably - pods or stacks will really help a "built" engine breathe. I would go with pods on a street racer. They are a bit quieter than stacks and the K&N ones are pretty good filters.
