This is about ubiquity and interoperability not security. All telco's are regulated and many of them have high data privacy standards that limit both what they can do with the data going over the network and who has access to it. In many countries this means a warrant must be obtained by the authorities. Far from perfect but good enough for a notification that the plane is late or a parcel has been dispatched.
It was the ubiquity of SMS over the D-channel that made it successful. Then the telcos decided they could charge for it and stifled further development and left the space wide open for the OTT chat services, which largely depend on collecting personal data and spurn interoperability. I think we all have at least two different messenging apps depending on who we're talking to.
WhatsApp is definitely the market leader and increasingyl companies are using it to contact customers, eg. booking.com and Deutsche Bahn. This is only likely to increase and I for one would rather have the communication running over a regulated system than at Facebook's whim.
The marketing and advertising side is being bigged up but I wouldn't expect this to change significantly because it mirrors existing services that are regulated. 15% is a great rate for any direct messenging campaign but you can understand that this might improve by being able to include a picture and a call to action, as we already have in a lot of apps. Being able to use standard infrastructure makes less development and maintenance work for companies.
So, all in all, there's a lot to like assuming the networks keep a lid on unsolicited messages.