Unlike LibreOffice, Google Docs has the advantage of being tied to a huge, well-known company that enterprise users are beginning to feel comfortable with. The problem with trusting any Web-based document editor is that you're entrusting your content to strangers. Obviously, this is where a local office suite such as Microsoft Office or LibreOffice wins.
Despite these privacy concerns, companies are adopting Google Docs as a viable option. Huge names in the enterprise space are making the switch—not to LibreOffice or other open source office suites, but to Google enterprise solutions instead. Based on this realization, I believe that Google is going to be the most likely player going head-to-head with Microsoft if MS Office makes an appearance in the Linux space.
Will enterprise users choose Google Docs over Microsoft Office for Linux? In the enterprise world, it's actually already happening. Bundle this with Google Docs being accessible offline through Chrome, and suddenly the comparison between the two office suites comes down to personal preference. Considering the fact that Microsoft would be incredibly late to the Linux market when compared to Google Docs, I don't think Google has anything to worry about.
Although Google is light years ahead of Microsoft in the office productivity for Linux space, some enterprise environments still prefer a physical office productivity product. And for some of those enterprise users, LibreOffice simply isn't full-featured enough to meet their needs.
I also enjoy the functionality offered by Google Docs. And unlike the Microsoft Office suite, Google Docs is available to casual users for free. While it's fair to point out that Microsoft Office 365 is a subscription service like Google's enterprise offerings, the fact is Google makes their tools available for free to casual users such as myself. This is an area Microsoft has yet to get a handle on and because of it, they'll finding winning over new Ubuntu customers a lot harder than they ever imagined.