I spend a lot of time without either signal or internet when in the countryside and this allows me to seamlessly communicate. The last thing I want is to either switch programs to carry on conversations, or manually choose what kind of message to send. Both would be a pain.
My main objectbbion to iMessage is that it only works with other apple products, so I end up using whatsapp with some people - where I need 2 apps to handle simple messaging and conversation histories get split up etc etc.