If the state can't prevent every murder, why would you expect that it could prevent every terrorist attack? Why does it make any actual difference whether deaths are simultaneous or distributed? Horrible things, intentional and accidental, happen every day, and they can't all be prevented, but you don't waste your time worrying about drunk drivers or lightning strikes, because they're spread out and you don't notice.
You make a reasonable and proportionate effort to protect yourself to the extent that you can, and roll the dice and pray to the extent that you can't. What makes terrorism any different than plain old murder?
To the first, it depends on what we have to trade away and on what we get in return. There will always be risks, because our resources are finite and the world is occasionally hostile. A 9/11 scale terrorist attack happening every few years is bad, but unless the resources we shift there make a bigger difference in reducing those threats they can make elsewhere, you should be "content" in the sense that that is the result of the policy that you should prefer. In this context, freedom/power can be viewed as an important resource: power in the hands of the security apparatus makes them more effective, but freedom in the hands of the people helps to protect against government abuses (in the extreme, tyranny - but things don't have to get there for it to be important).
Once you get into the business of having your government spend trillions of dollars and destroying other countries in response to a couple idiots, you paradoxically make terrorism more likely, because shifts in policy accompany it.
When no one gives a shit, there's no incentive for ideological terrorists to attempt any interesting attacks, leaving only the occasional madman.
"We spend $0 a year on defense against flesh-eating bacteria. Should I, as a citizen, be content with flesh-eating bacteria happening every few years?"
"We spend $0 a year on defense against maniacs going postal. Should I, as a citizen, be content with maniacs going postal happening every few years?"
It's still wrong to say we spend $0 on X, Y, and Z.
David Foster Wallace says it better than I could: http://www.theatlantic.com/magazine/archive/2007/11/just-ask...
Yes.
We spend $0 a year on defense against alien invasions. Should I, as a citizen, be content with alien invasions happening every few years?
Also, you don't know that DoD spends nothing against alien invasions. :)