I'll chime in in support of reinstalls.
I decided to try to repair an install (against my better judgement) due to recommendations from other techs.
I had the operating system and programs working fine. However, the HP printer driver frontend (using .NET) crashed on startup. No combination of reinstalling .NET, reinstalling the drivers, dial-a-fix, or re-registering DLLs by hand would make the frontend launch.
I promptly reinstalled the box and everything worked out fine.
At that point, I decided to always go with my instincts. If I feel a box is better off being formatted, I do, and don't think twice about it.
P.S. I didn't bill the client extra for my mistake. I never should have attempted to repair the computer. I billed them my standard reinstall rate and chalked it up to a learning experience.
Good call as I've stated before every time I've done a repair install it's took me 3 times longer to fix and I've ended up having to reinstall anyways.