When I first started smoking a pipe, I sometimes used the little, hollow paper filter that came with my Dr. Grabow, and sometimes I didn't. It did not seem to make a difference. My first few pipes I purchased after the Dr.G were filterless, so I forgot about those filters. Later, I ordered a Bjorne Viking on the internet, and it was a 9mm filter model. After smoking with the charcoal filter, I really liked it. I liked it a lot. The reasons were: I found that I never had to use a pipe cleaner while smoking, to absorb moisture. My pipe never gurgles when smoking even the most heavily cased blends. I believe the draw, being a little heavier, helps me puff slower. My pipe doesn't seem to heat up as much. That, helps me get less tongue bite.
Even though I still use my Churchwarden and Aldo Velanis without filters, I find that blends taste the same. I still get the nuances of each blend.
I have never known another American smoker, who uses a 9mm filter, but I read that over 90% of German pipe smokers use them. Of course, it's all about: To each his own. I do save money on pipe cleaners, but those charcoal filters cost money. I am not a filter salesman. Any thoughts?