Not having to rely on the good intentions of Signal or the corporate interests of WhatsApp/Line/WeChat/Telegram/etc. is a good reason in my book. There's no proof of bad intentions, but if I were the NSA/CIA, I'd set up a service like Signal, tweaked to encrypt in such a way that only I can decrypt its messages.

SMS/MMS is simply terrible to use, but at least it follows the normal "my carrier sends messages to your carrier" approach. The alternative "my carrier sends messages to Facebook to send messages to your carrier" flow adds an unnecessary middle-man, most of which will sell your data.