You know, I think any sport where the sole intention is to punch someone else in the face has its merits as a form of self-defense. In my personal opinion based on the one fight I've been in outside of boxing I think that any combat sport training can really help you. For one you get a lot of experience with fighting since all combat sports incorporate sparring, you also learn proper technique and will most likely just be more experienced and skilled than the average person.
I do get baffled by people saying that "boxing wont work in a street fight" since I can only assume it's a lot of wild punches thrown with reckless abandon, telegraphed punches, and no display of any competency in striking. Can a sport who's very foundation is punching while avoiding punches really be ineffective in a scrap?
If you were walking around Liberty city and a person comes up to you looking to assault or rob you would you rather punch him and leave the area or go to the ground and grapple with them?