I came across this post earlier in the week - https://gigaom.com/2015/01/18/why-sms-is-the-new-channel-for-customer-support/ which discusses how using SMS for apps is under used.
I think it may have been a sponsored post, so slightly biased, but we all now that SMS has a higher interaction rate than other communication mediums. I would say you can call SMS a ‘messaging app’ of sorts, just a very old one.
Taking SMS alongside all the other messaging apps that keep getting released, what would you say is missing and you would add as a feature if developing a messaging app?