It also gives your client a few less seconds to notice the embarrassing personal email or IM banner in the top right of your presentation.

Apple doesn't seem to have thought of this, even though we've all seen personal IMs appear in popups during presentations. The solution is not to have those events appear as banners or alerts, but then they don't appear in Notification Center at all. I'd like to use Notification Center to check what emails, IMs and calendar alerts came up while I was away from my computer, but I can only do that if I want them displayed during presentations.

Oh, I'm griping about Mountain Lion again.

Is this a hypothetical situation or did it actually happen to you? Apple did think of this:
- alerts and banners turn off automatically while you mirror to a projector
- for exceptional circumstances you can pause banners yourself using the switch at the top of Notification Center

A Google search will reveal more info or you can check out
http://thenextweb.com/apple/2012/07/25/tnws-complete-guide-to-notifications-in-os-x-10-8-mountain-lion/