Good post Michael.
Here are my thoughts:
"One interesting argument that several people have stated is that if you don't spar, you can't fight; or, if you don't spar, you will have limited success in street self-defense."
There are a lot of really tough people out there, some with no martial arts experience per se, and that obviously don't spar.
"I would wager that any good black belt in our system will have no problem defending himself whether he spars or not."
Probably true but would he/she have a better chance on the street if the individual fought regularly with contact against good fighter? I would say, yes.
I fight with my advanced students and black belts every week for several reasons: it keeps me in good shape by building my endurance; it helps my footwork; it teaches me how to fight against people of all sizes and shapes; it teaches me how to hit specific targets with specific weapons when someone is attacking me in a way he/she chooses; it keeps me humble, especially when I take some vicious shots (LOL); and it teaches me how to take a shot, and give one (or two or three...).