Another possible goal of terrorists could be to bring about an atmosphere or force a narrative. For example, by attacking a country, it might make it easier to recruit when the inevitable cultural backlash results in increased hate-crimes and racism toward the minority.

I imagine that this wasn't one of the goals of the IRA, but it looks to be a major motivator for ISIS, which is trying to recruit from the vast majority of peaceful Muslims in the world with a narrative of West-vs-Islam. If even a very small number of Muslims are radicalised because their mosque was burned down or their mother assaulted, for example, that's still a win for the terrorists.

Due to the asymmetric nature of terrorism, which after all is the whole motivation behind using terrorism in the first place, I think it's useful to apply some of the same thinking we do when considering the payout for spam email - if one in 100,000 buys the product / falls for the scam, it's a success.