I see where you're coming from...
A clean install in both Windows and Mac = good.
Re-installing all your programs, data etc = bad.
'Archive and Install'
Apple therefore created 'Archive and Install'. This installs a brand-new, clean version of the Mac OS to boot up and use, but it keeps all your user-data, programs etc untouched. The 'old' OS is put in a directory called 'System (old)', just in-case you need a file out of there. Once you've ascertained you don't you can just delete it.
This is the best of both worlds IMO, you get all the benefits of an erase and install without all the hassle of actually having to do it.
Tips for Clean Install
If you want a clean install when upgrading from 10.5 > 10.6, then I recommend:
1. Backup everything to an external drive.
2. Upgrade your system first
3. Boot up, and open all programs that contain databases, e.g. Mail, iCal etc.
4. You'll notice very often between major versions Apple changes the format, so these programs will 'update' your user data to the new format.
5. This is a good idea because when you do a clean-install and then restore your data, you aren't immediately cluttering it up by upgrading these. It's a minor benefit, but that's the whole purpose of a clean install anyway. Also it means if you have to grab a file from your backup drive at a later date, you don't have to worry about it being in the old format.
AppleMatt