My understanding is that there is actually little correlation between nasal cultures and the responsible organism in sinusitis and pneumonia. Additionally, the vast majority of sinusitis cases are caused by a virus, so this test would be meaningless. If you have ever performed gram staining or other techniques used to differentiate bacteria, you would know that it is not a simple 10 minute foolproof, bedside procedure. Even if it were, doctors are barely allowed ten minutes per patient per visit, so tacking on a lab procedure to every visit where someone complains of sinus problems would burden the system incredibly.