There is also another case which is finding out who did it - and in recent reports of police investigations there is always about use of surveillance cameras. The surety of being caught is also important in prevention. Maybe less in the case of suicide terrorists - but they are really a minority of all crime.
I have the feeling that people are so much against surveillance that they cannot stand any analysis of the arguments used. This is counterproductive.
Have you watched the news these last few days? Known extremists managed to get AK-47s and explosives in the middle of Paris, at the highest alert level of a military-on-the-streets Vigipirate program, while the borders were closed and the city's security was further elevated for an upcoming international meeting about climate.
They were known to law enforcement, they communicated over cleartext intercepted channels, they lost themselves in the noise of tens of millions of people joyfully filling some Stasi's storage units with their private conversations.
And all this less than a year since a similar event lead to the incompetents asking for even more data that they can stash for after the fact investigations. So there's a valid use: finding out, in retrospect, exactly how they failed to do their jobs. And maybe ask for even more funding, even more privacy invasions, even more power.
You also need to realize that people that were doing unpleasant surveillance in parked cars, at night, trying to figure out what a known arms dealer was up to, are now enjoying the comfort of a desk, looking at a database of intercepted SMSes that don't make any sense, and waiting for the shift to end.