I don't see why a phone manufacturer should be allowed to lock my phone number in their system in the first place. Why would i need to tell my phone manufacturer that I'm switching? What does he have to do with that? #justapplethings
Well if you are using a service they provide which is exclusive to them then yeah you should. In essence you allowed Apple to act as your SMS gateway/relay and when you enabled iMessage then decided to not use it anymore you should tell them so they can clean up their end.
Wouldn't you expect your cable tv provider to just know that you've moved to a new residence or would you expect to call and let them know you moved?
Obviously Apple did a poor job of telling people that they need to let Apple know they've moved on.