When I get the brand guidelines from a client (explanations about logos, colors, fonts), the PowerPoint section is usually at the back, put there as an afterthought after brochures, business cards, and letterheads are being discussed.
Designers usually do not pay much attention to PowerPoint (PPT is uncool for serious designers) and you end up with fonts, shapes, and concepts that are 1) hard to incorporate in presentation design software (no, most people do not have Frutiger installed on their machines) and 2) - more importantly - are very hard to understand for the layman designer.
The face of a company used to be the letterhead, but today it is the website, and yes, the PowerPoint presentations that are cobbled together by the amateur designers and shown to customers everywhere, all the time.
So, when designing a new corporate look, think about those amateur designers, and the best way to do that is to design your look for PowerPoint, then adjust it to other canvases. Sorry.