Although Macintosh hardware holds its utility far better than PC hardware, it's still difficult to justify keeping older Macs and accessories around through major architecture changes. Sure, a 68K Mac was still generically useful for a while after PowerPC Macs arrived on the scene, but when you added in the additional evolutionary change of switching away from ADB, SCSI, and serial ports to USB and FireWire, the argument for keeping that 68K Mac and all its add-ons became even trickier to make. That's especially true with any form of outside pressure, such as we experienced last year in having to decide to move all this elderly hardware across the country at a cost of roughly 75 cents per pound.
But I'm here today to recommend that you do keep old hardware around if you have the space and don't have anything better to do it with it. (That last bit is important - if you can sell or donate old hardware to keep it in frequent use, I believe that's better than storing it away for some unknown future application.) Here's my story of the utility of keeping old hardware around.
Our son Tristan turned three a few weeks ago, and we decided he was old enough to start using a computer on his own. This isn't something you do lightly if it involves the child using a new Mac unattended, because the likelihood of accident is just too high. But with an old computer, there's far less worry if a cup of milk accidentally destroys the keyboard. Plus, the software I wanted to start him out with were Broderbund's Living Books, which are basically just books with the addition of interactivity in the form of objects that do things when clicked, shape matching activities, options for free-form building, and so on. They've been around for quite a while, and although most haven't been actively marketed or updated in years, they work perfectly with old Macs. As is the case with a fair amount of educational software, you can still find them, a few from LivingBooks.com, from ClassSource.com (which had a number of titles not in their catalog for sale at a Macworld Expo booth; call them for availability), and on eBay (which turns out to be the motherlode of toys and clothes which either aren't currently fashionable or lack a tie-in with the latest heavily marked character).
So I pulled my old Centris 660AV down from the attic along with an NEC 3FGx 15-inch color monitor. I had to fuss with it briefly to get one of the VGA adapters from my drawer of such items (it also held the necessary ADB cables, old mice, power cables, and so on) to work with the Mac, but eventually I got it working at a resolution of 832 by 624. After a few hours, I shut the 660AV off while trying to change to 640 by 480, since the Living Books use that resolution and couldn't resize the screen with the setup I'd created. Then I couldn't get the Mac to boot at all, and after a few moments, I remembered that the date had been some day in 1956, and that problems with monitors staying black when an old Mac booted were often related to a dead clock battery (if you're worried about this happening to an old Mac that's in use, try Polar Orbit Software's PRAM Battery Tester, which watches for signs the battery is going and alerts you). I called a local Radio Shack, which claimed to have the battery, but when I arrived, it turned out they'd lied. I was depressed by this, since it was going to be difficult to explain to Tristan why he couldn't use the computer for the next week while I ordered a battery from anyone other than Radio Shack.
But then I remembered that another computer in the attic was an Apple Workgroup Server 6150, essentially a Power Mac 6100 in the same case as the Centris 660AV. It had a dead CD-ROM drive (which was why I hadn't thought of it for running the CD-ROM-based Living Books) and a slightly dodgy 700 MB hard disk, but I figured I could steal its clock battery. When I'd brought it downstairs, though, I realized I had it backwards. Instead of putting the good clock battery in the old Mac, I could move the good CD-ROM drive from the old Mac to the newer model. I did that, and it worked fine.
The only problem was that the 700 MB hard disk was loud, and although it hadn't been a problem when the 6150 was our primary mailing list server in a noisy machine room at digital.forest, I didn't want to subject Tristan to the sonic barrage. Another trip to the attic produced an external 1 GB hard disk, but when I disassembled the case, I found that it was a full-height drive that wouldn't fit, so back I went for the drive from a 2 GB external hard disk. Less than an hour later, the Mac was happily installing Mac OS 8.1 (did I mention that it's always good to keep old system software too?).
I won't bore you with stories of figuring out how to disassemble all of these devices, move the bits around, and put them back together, but I will recommend that if you embark on such a project, keep careful notes, since it's easy to forget how it all reassembles, particularly when you're dealing with tiny plugs and jumpers on hard disks. If you're not familiar with how to take any particular bit of hardware apart (or simply can't remember) there are many Internet sources of the necessary information that a Google search will reveal.
When all was done, I had spent no money for a fully functional Apple Workgroup Server 6150 with a 2 GB hard disk and working CD-ROM drive that plays Living Books (and a fair amount of other educational software for the future) just fine. The 660AV is even less functional than before, thanks to the dead CD-ROM drive it inherited from the 6150, and I wouldn't bet on the 6150's old 700 MB hard disk working well in the external case (especially since I couldn't hook up the SCSI ID selector), but the overall utility of that combination of hardware has increased. We went from two computers, a monitor, and an external hard disk taking up space in the attic to a great system that Tristan can use without constant adult supervision (and a few more boxes of spare parts). Although all that hardware was expensive when we bought it, it's had many years of constant use, so anything it can do now is pure gravy on top of completely depreciated hardware.
Hmm, I'd better watch what I say - with a three-year-old, gravy on top of the computer isn't impossible. But the moral of the story is that keeping old Mac hardware around (packed neatly in original boxes for protection) can prove to be an efficient way of extracting the most use - even initially unintended use - from seemingly useless hardware.