I agree that one should be prepared to use all media in the overall instructional context but a preflight briefing is an intensely personal thing which needs to be tailored to that particular student's strengths and weaknesses. Writing as one talks, whether on whiteboard or paper allows the necessary flexibility which pre-prepared stuff, be it powerpoint or OHP, does not.
On a purely practical level, writing words is foolproof. Time is always at a premium when briefing, so firing up the laptop and enjoying the occasional system glitch can be embarrassing.