Sorry to necro another thread, but I'm the first "Other" vote and the poll does ask me to explain...
I have Siege from CS&C but I also want the Besiegers, because the tile count is higher and also I think the Besiegers artwork is the best of the three.
I don't feel a need to track down the Cathars because I don't really like the artwork: neither the broken-down walls ruining the castle lines, nor the faces-in-circles ruining the landscape. (The Plague doesn't interest me either, for the same reason: Carcassonne is a very beautiful game, but The Plague is garish and unfriendly to the landscape.)