If you look at the ranking peer-reviewed research on pipe smoking and health complications over the past 15 or 20 years, many studies will break down pipe smokers by depth of inhaling...usually something like "none" "slight" and "deep". Since these studies are survey based, the standard sort of caveat applies that "none" usually means non-intentional inhaling...it doesn't mean that these people don't actually inhale, they just don't make a point of inhaling. As was mentioned in a post above, you can't taste the flavor (especially of an aromatic) without at least taking some smoke in--its simply biology. That said, slight (I think) would be those that intentionally either retro hale or french inhale (like I do...i.e., blow out the mouth and inhale a small portion of smoke through the nose); deep inhale would be what my grandfather did which was every 3 or 4 puffs, he would literally inhale through the mouth that last puff and take it in deeply.
All that said, there is an excellent study done back in 2004 that out of the pipe smokers in this study, 5243 of them responded "none", 1741 responded "slight", and 1091 responded "deep"...in percentage terms this would mean that roughly 36% consciously inhale, with roughly 22% of those being retro or french inhalers.
So, since this is a relatively large study, I would assume that the percentage of these in the population would be about the same.
(Note: of course there is sample bias going on here. I'm sure that some of those that responded "none" in fact do consciously inhale, they just aren't telling the survey taker that for whatever reason.)