The bigger headache is the iCloud account required to send iMessages. A pain to set up, and after a few spam messages Apple will just terminate the account. A

The bigger headache is the iCloud account required to send iMessages. A pain to set up, and after a few spam messages Apple will just terminate the account. And probably blacklist the phone used to do it. Sure, this isn't terribly expensive, but compared to the barrier of entry to SMS (zero), it is pretty substantial.