Actually, when I first came upon some of these pilots, I thought they were previews of actual upcoming shows, and expected to find some of them on the fall schedule. That might be another reason why networks stopped airing these things; viewers might have mistakenly assumed that these shows had been picked up, and (if the pilot was any good) feel disappointed not to find them again.
I suppose we all can remember one or two of these pilots if we think back hard enough. I can remember two, neither of which was any good. One was a Cosby-style family sitcom about a recent graduate who takes a corporate job that he hates, just to please his dad. His girlfriend gives him some lecture about how he shouldn't pretend to be what he's not, and so he quits the job, and argues with his dad about following his dreams and stuff, and they make up or something. I remember two lines of dialogue. One was the hero singing "I'm hating my job, I'm hating my life, I'm hating my job." The other was their idea of a great joke:
GIRLFRIEND: You did great things in school. You even got one of the Jacksons to appear at our prom.
HERO: Well, to be fair, it was Tito.
The other one was about a woman who marries into a family of witches. It was like The Cosby Show meets Bewitched, and the only line I remember is a bit where the Rudy-ripoff little girl tells the nice widower father that one of the sons doesn't want to go out in the rain:
NICE WIDOWER FATHER: Son, I told you, you won't melt if you go outside!
I suppose I could find out actual titles for these pilots if I consulted one of the books on unaired and unsold pilots, but somehow I prefer letting them live in my memory as vague but painful experiences.
There's also one other pilot I remember, about a bunch of people working as ushers at an old movie theatre called The Majestic. The plot of the pilot was that the theatre was going to be torn down, and the act break line was "They're gonna destroy the Majestic!" Everyone looks shocked, fade to black. I don't remember how they got out of that one. (Update: it turns out that this was actually a series, albeit one that only lasted six episodes.)
Any bad (or good) burned-off pilots that you remember seeing?