It's fair to say that Microsoft invented Web 2.0 with the original Outlook Web Access online mail service. It was the first prominent implementation of a Web front-end that had live links to back-end data and a user interface that approximated a desktop GUI. Today it looks primitive and, in fact, until Office 365, Microsoft did little to improve the interface. Worse, it worked much better in Internet Explorer than in other browsers -- and lots of users make a habit of avoiding Internet Explorer.
The Web apps are much better now. My experience with them makes me feel better about my decision to eventually ditch the Office 365 edition that includes the desktop software. I haven't found much that I wanted to do but couldn't. I could even print to local printers, something Google Apps can't do, at least not directly.
The Microsoft video below shows the PowerPoint Web app running on two separate systems, both editing the same presentation. Changes in one are reflected in the other.
Slick as this is, it's not like Microsoft invented it. Google Apps was doing it from the beginning, in 2006. The Google Apps applications, especially the spreadsheet, started out quite primitive and almost useless. From what I've seen recently, they have improved a great deal. There is at least some support now for pivot tables and I've been able to import spreadsheets which, years ago, were too complex for Google's spreadsheet app to make sense of. From what I can see, the main weaknesses are in layout, where Office has many more and finer-grained options. That said, it's harder to dismiss Google Apps these days. Just look at the side-by-side of the Excel app and the Google Apps spreadsheet pictured here.
But perhaps the question is moot. For a business, the ability to use Excel, including the desktop version, is a great selling point. With Google Apps, larger businesses are bound to run into limitations I didn't see, especially when dealing with legacy documents.