While I'm absolutely opposed to PRISM and similar programs, I think it's a little disingenuous to suggest that the only potential value was from stopping terrorist attacks. Look at this leaked powerpoint slide:
If you want to argue against PRISM-like programs, it would behoove you to tackle their actual purpose. We shouldn't just argue that PRISM isn't good at catching terrorists, we should argue that the total value of intelligence gained from it is not worth sacrificing our privacy for.
https://en.wikipedia.org/wiki/File:Prism-week-in-life-straig...
If you want to argue against PRISM-like programs, it would behoove you to tackle their actual purpose. We shouldn't just argue that PRISM isn't good at catching terrorists, we should argue that the total value of intelligence gained from it is not worth sacrificing our privacy for.