I could be mistaken, but I believe that this file is as it's name implies. It will enable you to uninstall SP2 if needed. With SP1, each patch had a separate uninstall file just like this one.
Someone with more knowledge on this subject will chime in sooner or later, but I wouldn't delete just yet.
In my WINDOWS folder there is a folder $NtServicePackuninstall$ which I assume contains the stuff SP2 set aside before installing the new stuff. Is this correct? If yes, is it ill advised to delete this folder?