They are only removed if you have repeated throat infections, if the tonsils are persistently infected or otherwise diseased beyond recovery they will take them out. It's not a very common practice these days because it has been recognised that the tonsils play an important part in the body's defence against infection.
Lovebugg said it all. Mine were removed at age 17 because they were always swollen to the point of difficulty in swallowing and mouth breathing. It was a symptom of other ills. Glad to hear it is less common to remove them.
Tonsils are an essential part of your immune system. They are masses of lymphatic material situated at either side at the back of the human throat. They are the immune system's first line of defense against ingested or inhaled foreign pathogens. However, their role is not fully appreciated or understood. Tonsillitis is a disorder in which the tonsils are inflamed (sore and swollen). The most common way to treat it is with anti-inflammatory drugs, or antibiotics. Many sufferers treat it by having their tonsils surgically removed by a tonsillectomy. This is basically unwise, because the primary defense of your system has been removed. Infections then immediately go deeper into the body and cause much more disease than if the tonsils were doing their job. BEFORE tonsils are removed, the doctor should be raising the patient's immune system so that infections are contained.