I think in general models can handle their own validation (especially if you use AR), but I guess the difference is that these validations hit external "service

I think in general models can handle their own validation (especially if you use AR), but I guess the difference is that these validations hit external "services", instead of like a :max_length validation. I agree on the first pass comment - I imagine that the user stories went something like: * v1 - An anon user can post to the guestbook * v2 - Guestbook comments are checked for spam * v3 - Post guestbook comments to twitter for Web 2.0-ness And this is the refactored version after v3