Almost, but not quite impossible! I flew out of Detroit on a Thursday morning and returned from Boston on Monday afternoon. And I recognized quite a few people on my return flight that were on my flight out, so there were a number of us who had a long weekend for less than $21 (airfare, anyway). However, I will say that in six months of subscribing to Spirit's e-mail, this is the first time I found the cheap flights that actually made a long weekend and went someplace I wanted to go. Also, it helps that Detroit is Spirit's main hub.
I think the real bait and switch comes from the high likelihood that people cancel these trips because they can't get time off or change their mind. When fares are that low and so hard to book, you tend to take a "book first, ask questions later' tactic. When I booked my Boston trip, I had no idea if I was going to be able to actually use it. I figured that if I couldn't, I was only out $20.82, which I thought was a reasonable risk. If I'd cancelled, Spirit would have still made a profit off me, since they wouldn't refund the money, but they also wouldn't have to pay the airport the fees of $20.80, and then they could turn around and re-sell my seat at full price. It is a bit of a shell game, but when it works out for you, it can be an exceptional deal. You have to be flexible and be willing to shoulder a certain amount of risk, though.