I think it's a wrong strategy to defend that the data did not prevent the subway attack or for that matter whether it's useful or not.

You are standing up for liberties and constitutional rights. And you can't have it both ways.

It's a good test for anyone against NSA collection:

If NSA proves that the data did stop terrorist attacks, will it be ok to intrude privacy then or put it another way are you ok with a terrorist attack on your country in exchange for your privacy ?