That's not quite right, you can't use any other MMS/SMS option with your phone number. It's a problem to say the least, you're pretty much missing out on a lot of services.2. You can freely use whatever options.
Now if they did allow making something else the default MMS/SMS client, that would be a GOOD thing. I'd switch to an RCS client in a heartbeat for more communications cross platform.
But we both know Apple wont do that, so no third party default MMS/SMS client. My bank (and others) doesn't offer notifications via the other third party messaging apps, only MMS/SMS -- so I *can't* turn off iMessage if I have an iPhone. That's the definition of a monopoly. And to tell the truth, I'm getting || close to saying the heck with it, I don't want an iPhone, it's way to stifling and controlled by Apple -- and soon after that, my other apple devices will go by the wayside. I already consider android superior in just about every way, it's only a matter of time. I'm only keeping it for an app that I need for work, for the time being. (It's available for android, but getting the data across is pretty much impossible, and that's Microsoft's fault, not Apple. I can do it a piece at a time, and already started moving it)