Why should I choose to implement Exchange 5.5 when Microsoft Office works for my company?
Well...Microsoft Exchange Server is a very different product from Microsoft Office. Office is an applications suite, some of which can be used as a front-end to Exchange server—Outlook most notably. Exchange Server is a back-end, groupware, and messaging server.
However, they're all just tools. If your company is satisfied with what it has, then maybe there's no reason to change. You have to look at the expected benefits and what problems you'd be solving. If you can't come up with a reason from those questions...then stay where you are.