It used to be that if you had lots and LOT of pictures, iPhoto would slow down, and you'd be told by some well-meaning soul to start a new iPhoto library. The problem being that to switch from one library to another required you to either dig up an app like iPhoto Buddy, or to learn the timing of holding down the "option" key as you started up iPhoto, so that you could select an alternate library.
In short - while less of a pain than dealing with iPhoto taking forever to do anything, it was still painful to do it manually.
It turns out hte latest updates to iPhoto 11 now allow you to switch to another library from within iPhoto.
While I still use iPhoto Buddy, it's nice to be able to switch on the fly without restarting iPhoto.