Per call, it is probably less expensive than the ship's phone, yes.
I don't know about renting one - I have never done it. I think if I did not have one, I would probably not make enough calls to justify the expense of getting one just for a cruise.
If you are a sailor, or a backpacker, or mountain climber, etc., where you often venture far afield with no cell coverage, then it probably would be a good investment for you. But just for a cruise - most of those calls can wait until you get home. In a real emergency, the few dollars for the phone calls will be immaterial.
And, you can always use the internet on the ship as a way to keep in touch.
Just my opinion - as I said, if I didn't already have one, I would not go to the expense of getting it.