On another note, I love them because they allow for limited info without keeping all your players in the dark. I've been in a game or two where the DM and a player where exchanging notes faster than I could keep up with and I felt a tad excluded, but with spoilers I can communicate to a specific player, but the others can get in on it too unless I specify otherwise. Gives a more inclusive, table-top sort of feel in my opinion.
Of course, that's because I think the players should be able to know certain things that their characters don't, but that's just my personal preference.