I don't think there are very many "terrorists" period, but the few that actually exist I think are quite determined. There are plenty of people who are quite prepared to kill themselves. Even eager. A dog isn't going to stop them. And they certainly aren't going to be afraid of one no matter how large and ferocious. If you have no fear of death I think it's pretty reasonable to assume that you won't be too afraid of dogs either. I don't think a suicidal person has any sort of "delicate mental balance". It is a stable condition. They have committed to dying for a cause that they believe in. What they consider a noble cause. If they are discovered by a dog they can just blow themselves up, preferably in the most crowded area possible. A busy checkpoint could be even better in terms of casualties, fear, and potential political change than an airplane. I can just imagine that they would love to see the effect of that on security procedures. Pre-checkpoints would surely be the result. And then pre-pre-checkpoints ad infinitum. It would almost be funny if it weren't so ridiculous.