Yes, I'm not saying there is no harmful software, I'm saying these kind of "protections" are the wrong solution. The same people who install blindly any add-ons will also install any exe-files if the promising website tells them to because the browsers add-on-system does not provide the required mechanics.

So the next step is to disallow exe-files. But of course you can let the exe-file get signed for a "small" fee...