I can see why it'd be valuable, and it's probably done a little to help out with counterterrorism since the program was still going on when the story broke, but we could make our streets safer by having two policemen in riot gear on every street corner beating suckas down for the most minor of crimes too. Sort of an unfair comparison, but the exaggeration is necessary. That's the philosophical part of it. Just because you can do something doesn't mean you should do it.
Metr0 pointed out the biggest problem I have with this though, and it's that it's illegal thanks to their own mysterious stubbornness/stupidity! Just get a freakin' warrant afterward if that's what the law dictates! They have what, two weeks to get a warrant after a pre-emptive wiretap? There are thousands of people working for the government, couldn't someone pop down to the courthouse for a minute?