i don't know that microsoft office is necessary, but i just thought some of its features would be helpful. in addition to some of the features of onenote described in the link i posted, here's an old thread with a more detailed discussion of microsoft office vs openoffice/libreoffice:viewtopic.php?f=5&t=152792
the docx compatibility is no longer a problem. libreoffice opens .docx files just fine now. so my only remaining concerns are:
1. the lack of onenote in libreoffice
2. a possibly better citation system in microsoft word. this could make your life easier when you're writing a paper with 50+ citations?
3. differences in the way documents look when opened in libreoffice vs microsoft office.
4. special features of microsoft word like adding comments to the document that may not be compatible with libreoffice
i don't know enough about other possible differences to comment further, but i'm sure at least some people here have used one or the other. please share your views!