Obviously we differ in our opinions.
I've had so many HDs go south on me, that I am really cautious in this regard.
Also, I do not believe in Smart Cloning, incremental backups, and the like. Why because they have all failed me in the past.
I only do full complete clean backups now and life is grand!
BTW, there is no casual or home use IMHO. Why? Well, have you DL'ed music or videos from iTMS? Have you DL'ed software applications (commercial type that you pay for)? Have you DL'ed other important apps that you've had to customize and configure? Do you have personal pictures, keepsake type, on your HD?
My point is this. Our data on our computers is becoming more and more valuable as we move to digital forms. Just imagine that if you lost everything on your HD. How long would it take to re-create your work environment and all of your files. Sure you may have them backed up on CDs/DVDs. Have you edited your photos for example that are in your iPhoto library? All these changes would need to be recreated if both your external and internal HDs went south at the same time. Call me over cautious, but having been there numerous times (internal and external HD fail) on PCs and Macs, I tend to be a bit overzealous on backing up my data these days.
External HDs are cheap these days. They allow for quick and easy backups. Get 2 of them!