I do it when I am really trying to pull the flavors and taste deeply into a blend. Having two pipes with different tobaccos really helps, because I can have one of my "I totally understand" tobaccos, like a C&D Virginia Flake in one, and the other can have some other mixture, and switching between the two gives me a deeper insight to the new one. When I return to the one with a new blend in it, it is like I get a fresh taste on it. Hawky (the guy with the black dog avatar) first suggested this on here, and I have picked it up when exploring something new.
I mean, I am not just walking around all day with two pipes hanging out of my clench, but I can see some benefit to doing this every now and then.
But, if we took a poll, I'm sure everyone on here would rank me as one crazy motherfucker, ha ha. But, not for this. They all just hate me because I'm so beautiful.