Larry Roush, a pipe artisan I really respect uses a bowl coating. When GLP reviewed one of Larry's pipes, here is what he said:
"But, wait! This bowl IS coated! After all my ranting and raving about how much I hate bowl coatings, about how the best of them are acceptable at best, I was quite surprised by the transparency of Roush's. What distinguishes this coating from the others is its complete absence of coloration to the taste of the tobacco. I asked Larry about it, about what it was, about why he uses it, explaining to him my usual prejudice against the stuff.
He doesn't use it to disguise flaws in the bowl, as some makers seem to, nor does he use it to "protect" the bowl from abusive smokers. It's raison d'être is simple. Because of his curing process, Roush's bowls are extremely porous. When he stains them, the stain sometimes soaks through the wood, into the bowl, leaving a mottled appearance. "It just doesn't look right," Larry told me, "so I developed the coating. It is 100% edible, and really only serves to improve the aesthetic appearance of the inside of the bowl."
I'd have to agree. It certainly does nothing to detract from the excellent smoking qualities of the pipe. And, obviously that porous nature of his wood plays a very definite role in the excellent smoking characteristics of the pipe. The wood coupled with excellent engineering, a wonderful, open draw, and perfect construction delivers to the smoker an extraordinary experience. From the first bowl, the tobacco has smoldered cleanly, effortlessly, right to the bottom, leaving only a dry ash, and the desire to smoke the pipe again right away. In fact, I did just that. Three times the first day. Each bowl was even better than the last. Break-in? What's break-in?"
See
http://www.glpease.com/Pipes/Reviews/Roush.php
So maybe it depends on the coating?