I was at a client site with my MBP when someone dumped 20oz of water on to my keyboard frying many of the keys. After trying many techniques to dry it out (with partial success), I ended up buying a new machine. Luckily, I had a very recent SuperDuper! back-up sitting on any external hard drive. Expecting the worst, I hooked up the new machine and started the restore process. 45 min later, I had an exact replica of the old machine. Exact down to the desktop image, the saved passwords, EVERYTHING was exactly like on the old machine.
This probably saved me countless hours/dollars of time.
Do I think back-ups are worth it? Yep.
These days I run a Drobo with 2TB of storage hooked to an Airport Extreme.