Well, to be fair, the crashing has gotten better in the latest unofficial builds, although it's still not what I'd call stable. Keeping an old Mac around is obviously a workable solution, but this takes up extra space in your apartment and can be a pain if you want to transfer files back and forth between the OS X and Classic environments. You've also got the problem that parts of the old Mac might start to deteriorate as it gets old (particularly the screen if it's a laptop).
If you do go with keeping an old Mac, make sure not to upgrade it to Leopard, because for some &@#% reason Apple decided to remove Classic support even on PPC Macs that should be able to run Classic just fine.