just a different view...
using the analogy of an automobile, you can:
1) buy one (new or used) and run it until it drops dead
2) replace it yearly, just for a change
3) rebuild the engine every year/month/week just for fun
Personally, my systems are here to be used for work. I maintain them
as needed, but using a good defrag periodically, saves me lots of time
(like waiting for the engine to be completed ~= reformat, reinstall)
Sometime reformatting is a last resort to getting back control from a virus
attack, but imo, that's an act of desperation and an admission that I did not
exercise proper control in the first place.