In my own case, the Macs are getting used...
My wife and I use our mid-'07 iMacs heavily every day and yes, I will admit that each of them have needed their internal hard drive replaced--once. They were covered under AppleCare. They're both valid for use with Mountain Lion when it comes out next month so we will continue to use them at least until the next big cat comes out. But that doesn't cover the other three.
The '05 aluminum MacBook still runs well, though admittedly we did need to replace the battery last year. Wife had a habit of leaving it plugged in all the time. She's still able to do everything on it that she does with her iMac, though she is limited to Snow Leopard as the OS. It hasn't slowed her down. The white, G3 iBook also still runs--limited to Tiger but now loaded with Ubuntu--and never used because it just doesn't have any purpose to us as a Linux box. It's the wife's play-toy as she keeps up with the Linux distros. Last but not least we have a G4 Mac Mini that for the last 4 years has served as a DVR and now that it's been replaced by an AppleTV box will turn into a local web server for our 5 websites. The iBook and the Mac Mini have never been opened for service and the iBook is still on its original battery. Of those three devices, only the iBook sees minimal use.
Hoarding? We currently have about 5 home-built PCs sitting in closets and the basement always with the thought of rebuilding them to modern standards--and never going anywhere with that. Our Macs have just been too reliable.
