NEVER is a long time
I suspect your advice was given by someone that had a partial picture of information.
Namely that viruses, trojans etc can live there, just waiting to be resurrected.
But if you're careful, you wont have any viruses etc on your drive, and therefore not in the restore because you're all up to date on AV, anti-spyware, firewalls, windows updates etc etc etc.
They can be extremely useful if something goes wrong with a program upgrade or install.
