Should We Accept PRISM to Avoid 9/11+?

How severe would a disaster have to be to make it worth putting up with PRISM in order to avoid that disaster?

How about 9/11? A 9/11 every decade? Every year? Nuking a major U.S. city? A major biological attack? Or is would no plausible attack be bad enough to warrant such a program?

I have no answer to this question, incidentally.