Sure OpenOffice, LibreOffice, Google Docs and the like are fine if you're only using it yourself or collaborating with others using the same software. I've tried mixing it with Microsoft Office and they lose all their value for me. There are simply too many inconsistencies or missing features in most cases.
If you rely on MS Office software then this $100/year deal is not too bad with the number of machines you can use it on. They're making it much more attractive than the one-time purchase versions with these additions. You can easily see the benefits for students or small business with these packages.