Another note: one thing that bothered me about the IBM Privacy and Security day set up was that the chair of the day asked everyone to hold their questions to the end of the speaker’s timeslot, when a few minutes have been explicitly allotted to Q&A.
This is a bad idea in general. Questions are a speaker’s way of figuring out what the audience likes or dislikes, is stuck on or understands. Questions allow for a dynamic presenter to tune his presentation to his audience. They allow the audience to ask richer, more useful questions with the relevant information fresh in their minds.
The good thing about holding questions until the end of the talk is that the chair has a better mechanism for making the talks and the session end on time. If the speaker ends on time, take a few questions, otherwise only take one. Given a mid-session question, a good speaker will be able to adjust the length of his delivery appropriately. But let’s face it: there are more bad speakers than good speakers.
I think mid-talk Q&A is worth it if the chair of the session is bold enough to warn speakers of the time remaining (through some visual cue, like a soccer warning card) and to cut the speaker off when time elapses.