Hmm...the tobacco doesn't clog up the pipe per se...it's the mixture of condensation and the tar/ashes that would clog up a pipe during smoking.
Condensation happens when the briar pipe is burning too hot, due to either saliva re-entering the pipe from the mouthpiece during smoking (happens to me sometimes when i'm clenching the pipe in my mouth. I hear the salivation goes away as one grows older...the body gets drier...a sign of bodily wear and tear), or from the tobacco being too moist.
But after every smoke the pipes are cleaned with pipe cleaners dipped in white rum.