"PDF Generator got no configuration, where no native application needs to be installed?"
Yes it does. If you want to convert images (jpg, tiff), or postscript files you don't need native applications.
"What is then the additional benefit of using LiveCycle PDF Generator ES?"
Automation! You might need to automatically convert a word doc into a PDF to start a review, add security, save for archive, ,etc.
"Would OpenOffice be able to convert MS-Office 2003 AND MS-Office 2007 documents?"
I believe it can.
Jasmin