In fact what I think it is nice in LR's Smart Collections is that they collect all the photos inside your computer which comply certain criterias.
If I want to find all my shots with the keyword "dog" and with for example red label, I can see them in a moment because the program finds all of them even in very different folders. Pretty useful.
Quick ! Efficient !
What pleases me in Bridge is the fact that it doesn't need to be Synchronised after moving or copying photos in another application. LR does and that annoys me.