"The majority seem to regard demo equipment more desirable than anything else, used. Could someone please tell me why?"
Its a safer purchase. There's all kinds of risk buying from private sellers. You're much less likely to have any problems when you use a dealer. Also, with a private seller, you have no idea how many times the component changed hands. If you've ever read some of the threads on this forum, you'll find that there's quite a few people who constantly buy used equipment to try it, and sell it a short time after. It may have changed hands 20 times, for all you know. Would you feel comfortable buying something that UPS/FedEx used for a soccer ball 20x? For a good description as to what I'm talking about, watch the beginning of Ace Ventura.
"I know there is a warranty but I have never had a piece break down in the first 2 years ."
Past performance is no guarantee of future results.