Any good Game Designer should be worth his weight in presentations, after all, one of the primary roles of the Game Designer is to sell the game idea, not only to external parties, but also internally to the team.
I remember an article that wrote that it is important for a Game Designer to be able sell how fun a particular game idea is to the team, as the team needs something to believe in and be convinced of when they spend weeks or even months before something tangible and playable is realized. And for the times in between, they more or less only have the designers' words to go by.
Presentations are vital to communicating game ideas, and pretty much go hand-in-hand with the Game Design Document, some teams even make their GDD a PowerPoint file, for easier reference.
Personally, I've done quite a few presentations, the biggest of which was during Gambit Media Day presentation when I presented Mūzaïc to the members of the press and reporters; and also when I pitched Troll & Armor to some big publishers back when I was with Emerging Entertainment.
Download Mūzaïc Media Day Presentation Slides
Download Gambit 2008 Game Design Test - Roy the Bouncing Hippo