For me, *manually* copying files and reinstalling programs over after a clean install is a major hassle, especially having to re-setup user settings.
I really don't want to spend hours resetting up my 14 email accounts and the 103 signatures I've got attached to those different accounts, let alone spending ages with different programs to reconfig the user menus/ribbons to be the way I like them.
I will wipe the hard drive and do a clean install of ML, but I'll then use a TM backup to bring over my files and settings.
This has worked better (for me) in the past than doing in place upgrades. I have done upgrades in the past, but found them to be buggy/slow. Clean installs and then copying TM files has worked well, resulted in a faster Mac, and saved a few gigs of space.
May not be as good as a clean install and a manual move of files and reinstallation of programs, but an improvement over a simple in place upgrade in my experience.