Well, the contradictory argument can be made as well: If you're a start-up, and your early adopters get annoyed because someone made sure Firesheep works with your web site, and they're all getting pranked, they're going to decide they won't bother.

OTOH, I am writing this comment on an open wireless router.

On the gripping hand, nothing I put here is private, and if someone "pranks" me, I can just login again and delete offensive content. Karma isn't actually money...