I think we're typing past each other, which is most likely my fault.
I don't really have a dog in the ADS-B hunt, but I can think of a couple reasons for discrete codes.
First, it can assist by giving out type of aircraft and normal speeds. You know what makes me concerned? Both the airports I frequent occasionally get business jets, and they don't fly the pattern. They sometimes talk jibberish, giving their position by instrument check points (and some sound a bit miffed when I ask them to state their position to the airfield). I'm coming around base to final at 50 mph and they're screaming in at 150. I have a tiny plane, and they might not physically see me or understand that I'm gonna take some time to land and turn off at the first taxiway....and I get a business jet up my rear or landing on top of me.
Second, and most likely, is institutional inertia. Transponders have unique codes, so therefore ADS-B does, too. No thinking required, because it's Transponder Plus, and so it flows naturally.