I have been involved with BX since 2003, and PaperBackSwap (PBS) since 2006. I really feel that there is a great deal to be gained by more interaction between individuals in the two groups, even though their goals are different but certainly compatible.
I'd love to add any BXers who also are PBS'rs to connect with me as Friends on BX and also on PBS. Likewise people in both groups who are also involved with Little Free Libraries. We have a lot in common and the groups really do complement each other.
I am a member of PBS as well as here. Personally, I don't mind if my PBS books have Bookcrossing labels. In fact, if I were able to post my Bookcrossing books there, maybe I could clean my shelves out a bit. Feel free to add me here, and there http://momrisner.paperbackswap.com/profile/
I am a member of both sites and it is indeed true that many members do not want bookcrossing labels (or labels of any kind) in their books but these tend to be people who expect brand new books from a swapping site. Many have a disclaimer right in their requests so I just decline them even if they don't have a BX label inside.
pjlareau is right. There are some good fits between the two. I trade some books that I have registered for books that are not often found at the thrift stores where I shop. So far it has worked out. I clearly state that all my books on that site will be registered here at BC, so far, no one has objected.