I've written enterprise software, used by large banks and other corporations. Our software was so bad, I couldn't understand how it would help anyone, I'm sure the people who used it were slowed down by the process.
Finally I realized they did get one thing from it: accountability. If you've never been there, it's hard to understand how corporations are shaped by SOX compliance, and general accounting problems. If a $2000 purchase disappears at a startup, it's a minor problem. But at a large company, accountants will be looking for weeks to find what happened to it.
Those are the kinds of issues large companies deal with, and removing the accountability of the decision making process (of figuring out what software to use) and giving it to Microsoft is a real service for them. This is the same reason people use RedHat, even though RedHat gives their software away for free. It is one of those things that makes no sense to you until you've worked in that kind of environment.