Well the argument against it has to be a moral and principles argument not a practical one.
I'm actually not amazingly sold on the it's ineffective line. If it doesn't in the state it's in now (not sure about that) ultimately it probably would catch "bad guys". Maybe not to the extent suggested or at a rate that is hugely more effective than conventional and legitimate surveillance methods but it'll work.
That is why the practical argument against it is at best a side argument and really shouldn't be used at all because it's irrelevant to why it needs to stop.