I can't see any use in downgrading - even less in installing one OS over another. It will end up just a big mess. Save data, format hard disk and do a clean reinstall. IMHO.
One application could be this, installed W7 Ultimate, decided I really only need pro or premium and can save money from not buying ultimate but I don't want to have to reformat when I buy W7Pro. I would say this "downgrade" option is pretty cool.
You are absolutely correct. This is the primary reason 99% of people want to downgrade. When you have been running RTM for months and have gigabytes of apps installed and your machine is running perfectly, no one wants to reinstall everything again when their is no need to. Downgrading is the solution and works perfectly.