First, the "we broke 20 plots" is bullshit. They have have used these tools in 20 investigations, so what? And what about the other 280 they admit to? And anyway, how many people's data was involved in each of these investigations? Dozens? Hundreds? Thousands?
In any case, we still come back to the basic problem: The police could certainly stop a few more crimes, if they were allowed unfettered access to people's homes. See someone suspicious? Walk in and search the house, no warrant required. The point is: This price is not worth paying.
Why? For many reasons, but here are the ones that leap immediately to mind:
(1) People need to feel they have personal privacy.
(2) Government bureaucrats are humans: some good, some bad, most just muddling along. Put this kind of power in their hands, and it will be abused. Whether for political ends, to get back at the ex after a nasty divorce, or whatever. Because they work for the government, they will not be punished. See the recent IRS scandals for a perfect example of this.
It is important to limit government power, because this is the only sure way to prevent abuses. You can't abuse power you don't have. If this makes police work a little more difficult, that is a price well worth paying. Convince a judge and get a warrant before spying on someone - this just isn't that hard.