Why the recommendation for the clean install? What is the benefit over the upgrade?
Simple answer is that a clean install not only upgrades the OS, but also removes all those mis-configurations, old unused (but still running) drivers for software and hardware, programs that have been deleted (but still left junk installed and probably running in the background), old versions of programs that you've upgraded but remain there since the upgrade software didn't remove them, old toolbars, that keylogger you don't know about, "helper" software that speeds up the loading for programs you no longer use, etc.
Basically a clean install is like getting a new car while an in-place upgrade is like getting a car that's new to you, but still has all of those old bugs and irritations you may have gotten used to, but hamper your performance.
What I generally do is replace the hard drive with a new one and do the clean install there, keeping the old one as a backup (and if things don't work as expected a simple drive swap puts things back as they were). Once things are working as I expect, I may (or may not, depending on what I need) add the old drive back as a secondary one so I can easily find the old files, etc. that I forgot to move over. The new drive probably performs better and has more space than the old one so you also get an extra boost.
