Unfortunately, people don't think about this until it's too late. I learned the hard way when my MacBook Pro's HDD failed unexpectedly, and I lost a lot of music, movies, games, code, etc. (had some of it backed up, though).
I'm glad Mac OS X and Windows include "set-it-and-forget-it" backup solutions, but users are reluctant to turn them on simply because they may not have the time or effort to set them up.
There's really no excuse for not using Time Machine on Mac. I think it's one-click setup (a yes/no dialog comes up the first time you plug in an external hdd to a fresh installation, if memory serves).
If you are afraid you won't be plugging an external hdd in enough, I think it's worth getting a Time Capsule (automatic Time Machine backup over wifi).
I'm glad Mac OS X and Windows include "set-it-and-forget-it" backup solutions, but users are reluctant to turn them on simply because they may not have the time or effort to set them up.