Earlier quoted context omitted.
My thought is that if we implement a back door policy, the public will have communication links which are much easier to compromise by any party, while terrorists can still easily use encryption and/or steganography to secure their communications. All the back doors accomplish in the end is harming the general public.
It would help conventional law enforcement. Most violent criminals probably don't put enough forethought into their spontaneous attacks to properly avoid notice by the NSA. Unfortunately the NSA doesn't seem to use their resources to secure the nation, and just ignores all the large scale violent crime that goes on daily.
I get that some of the controversy lately is about how well they stick to their mandate, but that doesn't change what the mandate is.