There are a lot of encrypted messaging apps but only iMessage is actually usable, plus Apple already has your billing info.
WhatsApp still doesn't have the new fancy encryption on iPhones, and you don't have to turn off SMS completely for iMessage; you just disable SMS fallback for users who are registered for iMessage
Source, please?
1. https://www.reddit.com/r/netsec/comments/34e6si/whatsapps_en...