I've only dedicated pipes these days for heavy ghosters, ie a cob that I only smoke with Big N' Burley and Picayune. And I wouldn't have done it, but those tobaccos demanded it. I could smoke paper shavings out of that cob, and they would taste like those two tobaccos. When I first started smoking, before anyone talked of doing this, I only smoked Escudo out of a certain pipe because it was the perfect size for two coins, and I found that to be the perfect length of smoke with Escudo. I wanted everything else I was smoking to last much longer, so it was about bowl size. It had nothing to do with isolating flavors to this wood or that.
And like GL Pease has written, I happen to like when the nuance of past tobaccos shows up in other bowls. They're a seasoning of the pipe, not a bogeyman. I was smoking 507-C the other day out of a pipe I don't smoke that often, and in the bottom quarter of the bowl, I got a couple minutes of a latakia hit, most likely from some old Crown Achievement I smoked in it last year. In experimental music, they refer to that as accidenz. Maybe if I was doing tobacco testing like Jiminks, I'd want to stay clear of such happenings, but I'm not. I find the unexpected like that to be fun, and I smoke for pleasure. Theory be damned.