From what I see, it 3G watchdog can't set data block to 0 because well, MMS is also data. Setting it to zero means turning data off, which can't be done because it will also turn MMS off. It should remain on. 3G watchdog will just act like a firewall where in when an app tries to access data, it blocks it. And the only app allowed to access data would be MMS/SMS app. I don't know how your carrier charges you, but this won't let you avoid data charges unless they charge separate for MMS. Some carriers in certain countries charge MMS as part of data if I remember correctly.