For example if we suspected someone was using Craigslist and other internet tools like Skype, to organize and coordinate a human trafficking ring, I would want them to be able to collect evidence akin to a wiretap to investiate the extent of the organization and bring prosecutions.
That after-all is the main thing, if the representatives of the people in terms of law enforcement/districts attorneys/federal have a reasonable suspicion that you're committing a crime, we should be allowed to investigate - and for you to challenge any evidence gained in a court. Previous abuses of power are of course troubling, the NSA phone tapping and others comes immediately to mind, which is why I believe energy is best spent on ensuring proper checks and balances and the rule of law - it requires constant work to ensure we have adequate protections for citizens and an adequate ability to prosecute suspected offenders.
Just a final point - I want to stress that I do believe the government has and will continue to overstep its bounds with regard to law enforcement, this has been true for time immemorial, and citizens will continue to fight back and push for legal reforms - but simply throwing our hands up and ignoring real, legitimate law enforcement needs seems silly too. Especially when agencies like mine can right now hijack individual sessions of you on our website and watch everything you do.