And sure, it's easy to agree that primaries for Congressional seats tend to be dominated by small groups of activists, and that this can lead to outcomes like Senator Ted Cruz. This is the dark side of that Margaret Mead line.
But if your best supporting example is that in a state without a "sore loser" law, Joe Lieberman was nonetheless able to get reelected after losing in the primary . . .
Well, I will just say that your argument is unpersuasive, Mickey Edwards, and so here is a cat riding a Roomba, wearing a shark costume, chasing a duck.
The problem is not extremists getting elected, because of the primary process. The problem is Republican extremists getting elected, and that is something the GOP is going to have to figure out for itself. Meanwhile, as you do point out, you can hold up Christine O'Donnell as a counterexample to Ted Cruz -- come the general election, democracy sometimes works.
Also, points to you, sir, for this:
For one thing, the political “center” is not always the right place to be (it certainly wasn’t in the pursuit of civil rights and women’s rights, or on issues like slavery and child labor). Compassion toward candidates who find their political prospects cut short is also of little interest to me.