"Well, weapons aside (this really is a moot point for self-defense anyway)"
Moot point? I don't know. Their presence does tip the scales waaaaay out of your favor but - never say die, until you are dead.
"BJJ does prpeare you to face any opponent. You may not feel that it effectively prepares you how to face an MMA opponent, but the odds of anyone facing an MMA type opponent on the street, are very slim."
As to what BJJ does and doesn't prepare you for, I can't comment. I don't do BJJ. On the surface, such a statement sounds like the statements made about karate and kung fu when I was a kid.
I will grant you that, being a grappling style, its much easier to see if you are doing it right in training. You do a takedown, either he goes down or not. You move to a position, either you get there or he stops you. You do a sub, either it works or it doesn't. That, IMNSHO, the any grappling methods greatest strength - instant feedback in training.
"Why would someone want to dedicate so much time and money, for such a small possibility? "
I'm not advocating training under the assumption that everyone is an MMA champion (although it does satisfy my sense of overkill). I do advocate strikers training against grappling attacks/striking attacks outside of their realm and grapplers training against striking attacks/grappling attacks outside of their realm.
Why spend the time and money? Some folks, like me, enjoy the activity so spending the time/money isn't an issue.