Because the cable is quite short, it is easy to tug on the connections when answering a call on a charging iPhone. I would rather the cable disconnected than put pressure on the connections or the cable. I have had a cable split where it goes into the connector because of this.
I have also nearly dragged my iPad off a table when I caught the cable. Magsafe is great, I would love to see it on iOS.
They made the magsafe 2 smaller on the retina and it has even less magnetic force. They would have to make it even smaller for the iphone or ipad. People also still use it to transfer data. What if it just pops off when transferring data. That would be frustrating.