Where people would rather use older technology than toss it.
I guess it's the world we live in today, but 90% of everyone I've ever known would rather throw out an older machine than keep it around. Granted, some vintage Macs are really beginning to show their age, but most of them are still usable. The two tricks are trying to convince people of that fact, and finding the use that best fits each specific machine's abilities.
For example, I have a 600mhz iMac G3. Sure, it's old.. and the PowerPC architecture isn't well supported by most current day applications, but that doesn't mean it's useless. For example, it makes a wonderful DVD player or basic email machine. If nothing else it offers a pure aesthetic centerpiece on my desk. The original iMacs really are quite beautiful, and are great conversation starters. Another example are iBooks and Powerbooks. While im not lucky enough to currently own any PBs.. I do have two iBooks. One is a G3 500mhz, and the other is a 1.33ghz G4 and both still have their uses. First off the G4 is a really nice all around laptop. It handles all normal internet surfing (including youtube) and unlike current day netbooks offers a built in DVD drive. The G3 on the other hand struggles on some webpages, but much like the iMac makes an excellent email box, and like the iBook G4 has the ability to watch DVDs.
On the craigslist Apple forum it seems everytime someone posts a question.. the typical response is "it's not worth messing with" One gentlemen goes as far as to always claim that a G model Mac's top value today is -15 dollars.. because thats what his local recyclers charge to haul them off. I am sorry, but that is the worst possible attitude to have about older Macs.
For many people, older machines such as the original iMacs or early G4 PowerMacs are their first experiences with Apples. Let's face it most people are unwilling (or unable!) to shell out over a thousand dollars for a new Mac if they've never experienced one before. For most people these older machines are their first taste of the Apple lifestyle. If we throw them away, not only are we getting rid of what could be totally usable machines.. we could also be limiting the chances of someone that's just trying to get their feet wet with Apples.