Are you referring to Microsoft Office specifically?
LibreOffice is a full-fledged office suite, is feature-complete for just about 99% of things you can do in MS Office, is free (both, as in beer and freedom), and runs very well under Linux.
Buy Office 365 Home and you get all that for five people, which could mean your family or flatmates.
You can't get all that stuff from LibreOffice. As a matter of fact, you can't get it from Google either.
Did you seriously just compare LibreOffice to MS Office? Look I'm a huge Linux fan, but buggy LibreOffice is nowhere near this imaginary "99% feature matched" you've spit out here. The performance is also a joke when it comes to LibreOffice.
As much as the open source camps would like to believe, Microsoft still hast them beat in the Office sector.
While I'm not saying a Linux port of Office would be just as fast as on Win8, it is a noticeable difference.
Why would it not be as fast on Linux?
I've used LibreOffice for years while running Linux before. I noticed perfomance issues compared to Microsoft Office on Windows that were especially bad for spreadsheets.
I consider myself to be a power-user of MS Office. Last time I used LibreOffice, it did not have many of the features that I used extensively (especially in their PowerPoint analog). In addition, the interoperability between MS Office and LibreOffice was not great.
Lastly--there isn't an adequate OneNote replacement on Linux. Evernote doesn't cut it for me. This is really my killer app--if there was a good OneNote on Linux (non-browser, a native desktop app), I could deal with the other issues as they came up.