They're just so very very wrong in this context, mainly because they're deliberately distracting. For someone trying to focus on delivering a presentation, this is 100% unacceptable. It would be tolerable (barely) if the ad never changed, but when the damn thing changes every few seconds, it's more than enough to break one's concentration. And that is LETHAL for public speaking.
I know HN guidelines discourage the use of caps for emphasis, and I almost never use them for this reason. But my negative reaction was SO strong that it's worth breaking protocol just to emphasize how awful those intrusions really are.
Aside from that one hideous, terrible, awful, repulsive, glaring deal-killer of a flaw, this app is brilliant. Bottom line: I'd pay $5 for a version without ads. I'd refuse to use it with ads.