For me if you keep on top of it there isn't going to be unneeded stuff to worry about. A format is a massive undertaking. Not just reinstalling an OS but all its security updates, in itself a monumental task. Then you have all the tweaks and settings you made in the time since you previously formatted.
Then you have all the programs to install, updates to download, tweaks and settings to apply. Setting up all Internet Connections, Network, registrations, user names, passwords, the list is endless and to me a real exercise of last resort.
This is also why I have hard disk imaging software, because on dial-up I would be just finishing off the security updates, etc. etc. when it would be time to format again, thanks but no thanks.
As to will your HDD last twice as long, perhaps not, inactivity may also have an impact on longevity if not stored in a stable environment, dust, dirt, heat, cold, etc.