The risk of not being compatible with XYZ Word/Excel/Powerpoint feature is too great. Do you really want your employees to not be able to open the client's document because you cheaped out on your productivity suite? Or, conversely, your prospective customer to not be able to open your proposal because of the same?

Alternatives did exist, who remembers Lotus Notes, Word Perfect, Open Office, Apple Pages, ..., but to use them today would be akin to deploying a new fangled AI web app as a Flash application.