Sure. When I was a freelance consultant doing Windows stuff, I did have a backup laptop for just that reason - in case an upgrade borked things, or my hardware

Sure. When I was a freelance consultant doing Windows stuff, I did have a backup laptop for just that reason - in case an upgrade borked things, or my hardware failed. OSX makes this particularly easy, though: you clone an OSX install with Carbon Copy Cloner or SuperDuper to an external drive. (I have this scheduled to run nightly) You can then boot another Mac from that external drive. The result is that you don't have to maintain two separate OSX installs. You can borrow somebody else's Mac, boot from your external drive, and boom - your data and your entire working environment are ready to go. Of course you could also accomplish this with other operating systems. If your main work environment is a virtual machine, it's pretty trivial.