Let's take an example out of my own history: Say you're the COO of a Corporation with several companies, all organized in a Microsoft directory structure (AD or whatever they call it) over several levels of hierarchy and also different places around the country.
Now you need to schedule a meeting with the CTOs of the companies, plus some of the CEOs, and maybe a handful other people from the administrations. Those people are scattered across the directory.
With Exchange (which I shunned myself before working heavily with it), I can just open my schedule, enter the recipients (with working autocomplete, of course I don't have them in my AB), say I need 1.5 hours of time, and press the free/busy button. I've just made an appointment in about one minute. This works on Windows, OSX and Linux, and is pushed to all mobile devices also.
Now, I am admittedly uninformed about the commercial GMail services, but I'd be amazed if they have free/busy functionality that plays even in the same league. And as the genius submitter probably wants to replace the Windows directory, too, I wish him all of luck, because I really don't think such functionality is easily replaced with F/OSS.
Don't get me wrong, I'm a big F/OSS proponent, but I can't imagine it can be feasible, both from a user and financial perspective. This guy just sounds as if he should find another job. He's already beyond his capacity, in several aspects.