The alternative could have been a permanent increase in military presence.
That said unjustified arrests are extremely stressful for the victims (those being arrested for no cause), so I certainly think that's terrible too.
C-51 gives law enforcement and intelligence some slightly more modern tools. For instance, they can demand that "terroristic" material is removed from the internet (where jurisdiction applies), which is a right that they already have for child pornography and hate material, with the agreement of a judge and the minister of justice. If they become aware of a subject, they can ask that the subject's ISPs capture data pending a warrant (versus waiting for the process of the warrant and perhaps miss things).
And so on. Remarkably few who comment on C-51 have any idea what it entails, or the scope changes it offers. But they ply, ironically, exactly the sort of rhetoric of fear that the right wing plies about terrorism -- somehow having an advance ability to request data capture == All lefties are going to be locked up.
Drumming a rhetoric of fear is exactly what is going on now to push this bill: http://www.cbc.ca/news/world/isis-recruiter-in-edmonton-enli...
and labeling inconvenient speech as "terroristic" is page 1 in playbook. This whole thing seems so ham-fisted that I'm amazed their are defenders of it.
Fascinating.
I think they're analogous to the NSA and FBI respectively.
For example, if you go to enough Animal Rights protests, I've heard that you start to get a "profile" in law enforcement databases (at least in the US). Just look at how "AgGag" laws are passed specifically to "protect" specific industries from activists that want to film what goes on behind closed doors (and bring it to light).
As another example, look at the way that law enforcement in Canada dealt with the G20 summit.