Great responses so far. @bobpyle mentions that he's hit audio nirvana and doesn't see why he'd have a second pair. Posters have mentioned a bit about why they like the variation which comes with a second speaker -- seems like tonality, dynamics, and soundstage can vary enough that there is a refreshing difference which can be introduced by a second set of speakers.
I'm pretty sure that there is a second set of speakers out there that would do greater justice to the symphonic music I listen to -- so that's another reason for swapping in a second pair -- to make another genre of music come to a fuller potential.