If you have the dongle you don't need the cable ---- do you get it now? You're swapping a little piece of plastic for a cable. You haven't made things any more convenient for yourself, in fact less convenient.
You only need EITHER a dongle OR a cable. The dongle is more convenient for the following reasons:
- Unless the cable is a very short one, the dongle takes up less space and you don't have to wind it up when storing it
- The dongle doesn't need anything extra - doesn't need the phone
- The dongle comes with some bonus software which you can choose to use or not
If you have the dongle you DON'T need the cable. The cable has NO features that make it more convenient than the dongle.