What do API gateways provide that @vtext.com, etc. addresses don't?
Guaranteed delivery.
The email to SMS gateways often drop messages, especially if you're sending quite a few messages through them. They might even block you if you exceed a (undocumented, completely arbitrary) rate. Practically, I wouldn't expect more than a 90% success rate sending emails through a cell providers' email gateway.
Beyond that, to use gateways correctly you'll ask your users for their provider, maintain a database of provider gateway addresses (which have been known to change in the past). Remember also that most of the world has number portability; this means you'll need to constantly monitor for delivery failures (from the gateways that even bother to send 'em) and ask users to update their provider.