First and foremost, I am not baggin' on filters.
That having been said, I avoid them as they (or at least the ones I have used) decrease airflow.
I suppose it comes down to the environment the system in question resides in, and the cleaning/maintenance habits of the owner.
If one has a tendency to clean and dust the system(s) weekly and lives in a house that has minimal dust, then filters may not be something needed.
I also have a medical-grade air filter that runs 24/7/365 so dust, pollen and general airborne detritus is constantly scrubbed from the environment.