Oh, I have heard about them. But there aren't that many people who are interested in blowing themselves up, and it is all but impossible to get any of them near the president (or any other highvalue target, but the president is probably the most difficult target that people actually want to get) - drones? Not so much.
They are even smart as real humans and don't care about their life.
The other point, terrorism is not abut effectiveness it's about publicity. Drones are not so much changing the game as it is right now.