One definite disadvantage of filter pipes is, that the shank has to have a certain diameter to take the filter, so all shapes will not be possible/available with filter. When I talk of filters, I mean 9mm (charcoal) filters, as 6mm filters usually have a high resistance and the taste detoriates considerably.
Smoking a filter pipe without filter, or adapter, is usually possible, but it is more difficult to clean the filter chamber, than with normal pipee. Some say, that more moisture accumulates in the chamber, due to airflow expansion, some others say, it smokes cooler, because of the chamber, but this depends on the pipe, the smoker and the tobacco.
Filter use is nearly non existent outside Germany, but I sometimes wonder why. Have you ever experienced a tobacco, which you generally like tastewise, but fries your tongue, or has a sharp or bitter taste, which you don't care for? Try it with a 9mm charcoal filter, preferrably from Stanwell, and the tongue bite is gone and sharp, or bitter tastes do not come through. It is true, that the filter is taking something out of the smoke, but this is not generally a bad thing. A tobacco with too much nicotine, but good taste, can be mellowed with a filter, too.
Regards,
Peter