Why not Gmail?
I was just surfing about our sites as you do and happened across this rather interesting little video that covers some of the reasons that Outlook is better for business than Gmail. As an ex Gmail user I wasn’t thinking about this stuff when it was just me using Gmail on my own but it’s hard to see how it can work well in a corporate environment. The amount of time a PA needs to be able to see resource availability or to securely (or in a SOx compliant way) access someone else's info is frequent enough that they’d surely get pretty frustrated pretty quickly. The need for BES servers to be on premise and for remote wipe to be a pain in the butt must cause lots of pain for IT Depts too. Anyway the video walks through some scenarios quite nicely and only takes a couple of minutes.