I thought of this argument after I watched this video:
MMA is not always indicative of the effectiveness of BJJ in a streetfight, b/c in MMA, there is often a reluctance to go to the ground (especially if it is known that your opponent has a nasty guard). Is it different in street fights (where the Gracies always said 4/5 fights go to the ground-which I am admit might not backed my the most sophisticated statistical data). Regardless, in a street fight, you don't know if your opponents knows bjj (unless you know him personally). And, more likely than not, in a street fight situation, your opponent does not know your game or tendencies. For the average Joe, strangers that might attack him don't know he has a ground game.
It also seems like with street fights, untrained opponents are more off balance and the fight is more likely to go to the ground. You certainly don't want to plan on fighting slouches, but, at the same time, in terms of self-defense I think it is good to consider what it would be like to fight an untrained opponent.
I have noticed that when I go to the beginner class at HQ. If I am going against a big/strong beginner, or maybe someone who fights MMA but doesn't nerd out on Jitz, they do not make some of the reactions I expect experienced bjj guys to make. It makes me realize that a certain % of my game is tailored to an opponent who knows the proper responses. For example, I hardly ever look to take the back when I get perfect double underhooks b/c I rarely get somebody who doesn't know to whizzer.
Now, I am not saying this will change the way I train, the only thing I have considered is making a beginner class every few months just to get a different look. I don't have to really go out of my way to do this, usually, every few months my schedule will dictate this for me.
But, it is something to consider. I originally got into bjj b/c I was worried about street fights.